To the Fun Science Gallery Contents

 

BUILDING A
STEREOSCOPIC MICROSCOPE

By Giorgio Carboni, November 1997
Translation edited by Donald Desaulniers, Ph.D.

 


 

CONTENTS

 


INTRODUCTION

What build a microscope? Why not! The following is a guide on how to build a stereoscopic microscope. This guide allows you assemble, at a relatively low cost, an instrument that is useful for observing natural objects such as insects, plants, minerals, fossils and flowers. It can also be used for home projects like examining art or antique objects, repairing watches and thin gold chains, looking for defects in electronic printed circuit boards or even help you find and extract an annoying splinter from your finger.
At first glance, the construction of a such an instrument may seem to be a particularly complex and difficult endeavor since costly precision machine tools are usually required build this kind of instrument. As you will see below, the approach we have adopted makes its fabrication relatively simple without having to use to expensive machine tools.
As a school project, the construction of this stereoscopic microscope can be a good exercise in optics, in mechanics (preparing drawings with CAD programs and working the pieces), as well as opening new horizons in biology and natural sciences. In fact, we have deliberately omitted the dimensions in the drawings provided in this article to simplify their presentation and to allow you to adapt the plans to the materials and components available to you. As presented, this microscope will cost less than a tenth of the price of a commercially available instrument of the same performance.


THE STEREOSCOPIC MICROSCOPE

So, what is a stereoscopic microscope anyway? In biological sciences, the two main types of microscope commonly used are the "conventional" type, usually referred to as the compound microscope, and the stereoscopic microscope. The main difference between these two types of microscope is that the compound microscope sees the sample from a single direction, whereas the stereoscopic type sees the object from two slightly different angles which provides the two images needed for the stereoscopic vision. The stereoscopic microscope gives a three-dimensional view of the object, while the same object appears flat when viewed through a compound microscope. This holds true even if the compound microscope has a binocular head because each eye sees exactly the same image.
Also, conventional compound microscopes are used to observe objects that are transparent or translucent and typically have a magnification ranging from 50 to 1,200 times. The stereoscopic microscope, however, views objects mainly by means of reflected light and its power, typically ranging from 8 to 50 times, is much less than the compound microscope. Conventional compound microscopes are often so powerful that you cannot even see the specimen with the naked eye. Stereoscopic microscopes, on the other hand, are generally not as powerful as compound microscopes. Although the stereoscopic microscope does not allow you to observe such small objects as microbes, there are nonetheless an amazingly large number of things to observe with this type of microscope.

While this apparent limitation of magnifying power may appear to be a drawback, the lower power does have its advantages. Indeed, the relative proportions of what you see with the naked eye compared to what you see through the stereoscopic microscope is maintained thereby making it easier to use. With the compound microscope you must often follow relatively complex procedures to prepare the specimens for viewing. This is not necessary with stereoscopic microscopes. The greater ease of use of stereoscopic microscope makes it particularly well suited for initiating children to nature and the sciences. This does not mean that it is an instrument for children only. Stereoscopic microscopes are widely used by researchers all over the world, in fields such as the entomology, botany, mineralogy, including applications like micro-surgery, micro-electronics and in may other commercial and industrial activities.

The simplicity and ease of use of the stereoscopic microscope is also attractive for the adult naturalist or amateur scientist who does not always have the time nor patience to prepare specimens for viewing. For example, when I come home, tired from a full day of work, I am not usually inclined to undertake complex tasks. On the contrary, I enjoy observing objects under the microscope just to relax. With a stereoscopic microscope, a short stroll in the garden will provide numerous and amazing objects for you to observe under the microscope. You can, for example, observe ants bringing out little grains of soil from their underground colonies, bees collecting nectar on flowers or even protozoa swimming in a pond of water.


BACKGROUND

As a young boy not having much money and with a great interest microscopy, I tried building my own instruments. I succeeded in making a compound microscope without much difficulty, but making a stereoscopic microscope proved to be a more challenging project. The commercial models I had seen in shops were made of two separate microscopes that were kept convergent as shown in Figure 1. The problem was not so much building the two microscopes that comprise the stereoscopic microscope, but to keep them perfectly aligned as you vary the interpupillary distance or the power of the instrument.

I pondered this problem for a long time, but the mechanical challenges proved to be too difficult and discouraged me from venturing any further in this project. For several years, the problem remained unresolved, until finally one day I came upon a brochure showing a cross-section of a stereoscopic microscope. I then realized that this instrument was not simply made up of two separate and convergent microscopes but rather an optical device in which the light passes through a common objective where it then follows two distinct paths. In this scheme, I reasoned that the common objective, which is comprised by several lenses, can be reduced to a single lens. This schematic drawing allowed me to understand that it was possible, and optically correct, to collect the two images needed for stereoscopic vision using a single objective. But more importantly, the problem of aligning the two separate microscopes, which is so important and difficult to maintain, was resolved. Indeed, as discussed later, this makes it possible to construct an orthogonal and parallel mechanical structure, instead of the convergent one of the original model shown in Figure 1 (A).

The next step in this project was to recognize that the optical components needed to construct the stereoscopic microscope could be obtained from a normal pair of binoculars. This is important since binoculars are widely available and can be obtained at a relatively low cost.

Also, using a pair of 8 x 30 binoculars as the "eyepiece" was important in order to simplify the project. At this point, many problems were resolved. Nonetheless, the question of adjusting the interpupillary distance puzzled me for a while. In the beginning, I thought of designing a stereoscopic microscope with a central pivot that could move together with the "eyepiece" of the binoculars. This device was puzzling and did not satisfy me. The idea of using the large sized prisms of a dismantled set of binoculars in order to contain the variations of the optical path solved every problem. In fact, simply moving the upper pair of binoculars will allow you to adjust the interpupillary distance. After having eliminated all other problems, the focusing movement is the main problem left, but, as you will see later, you can resolve it without great difficulty.


THEORY OF OPERATION

Let us examine how the common-objective stereoscopic microscope works. If you have a pair of binoculars, you can try this simple experiment. Unscrew one of the objective lenses and place it in front to the other. Approach an object until its image is in focus. You will see the object magnified. What is happening? Consider a specimen placed at the focal distance from the objective. The divergent light rays coming from the object pass though the objective lens and emerge parallel. This light then passes through the second objective of the binoculars. But binoculars are specifically designed to observe distant objects that are essentially the result of parallel light rays. In fact, the second objective forms an image of the object in the correct position of the eyepiece which makes it appear distinct.

In this simple experiment we obtained a monocular microscope, however we want to build a stereoscopic unit. To accomplish this, it is necessary to collect two distinct optical paths from a single objective and to pass them through the binoculars, which serve as the eyepieces. This can be easily done by means of four prisms as illustrated in Figures 2 and 3.

Let us examine more closely the optical schematic of the common-objective stereo microscope. The lower part of the microscope includes an achromatic positive lens and four prisms. You can obtain all of the required components from an old pair of binoculars (distal binoculars) which you can afford to dismantle. The upper part of the stereoscopic microscope comprises a second pair binoculars (proximal binoculars). This second pair is used without any alteration as if it were an "eyepiece". When the microscope is not in use, the later pair of binoculars can be removed and used for its original purpose: to view distant objects.

If you maintain the common objective at the focal distance from the object you want to observe, the divergent lights rays from the specimen pass through the objective and emerge as parallel rays which are then suitable to be observed through the binoculars which serve as the eyepieces. The four intermediate prisms, as shown in Figure 2 and 3, allow the proximal binoculars to view the two optical paths passing through the common objective.

This design of stereoscopic microscope is not only optically simple, it also has some important advantages from a mechanical point of view. The fact that light rays are parallel as they emerge from the common objective allows you construct a parallel and orthogonal mechanical structure, thereby avoiding problems of optical convergence and alignment as would be the case if the instrument were made up of two distinct microscopes as shown in Figure. 1. Moreover, the parallel arrangement of the two optical paths is well suited to the structure of the proximal binoculars, which is also parallel. These features simplify considerably the construction of this instrument without having to use precision machining equipment such lathes or boring and milling machines.

With this simple design, you can also adjust the interpupillary distance without any mechanical movement of the microscope by simply making the adjustment on the proximal binoculars since they already designed to do this without any difficulty.


In summary, the key features of this stereoscopic microscope design are the following:

- collects the two images required for stereoscopic vision from the same objective;
- uses a series of prisms which allows a pair of proximal binoculars to focus on the optical paths emerging through the common objective;
- exploits the relatively large size of distal prisms to allow the adjustment of the interpupillary distance without any mechanical movement of the microscope;
- the optical components are readily obtained from a dismantled pair of binoculars;
- uses an 8 x 30 binoculars as an integral part of the microscope.

Thus, what we now have is an optically correct structure that is also simple from a mechanical point of view. The optical design we have adopted makes the construction of a stereoscopic microscope reasonably accessible even to the amateur scientist working at home without any specialized tools.


CONSTRUCTION
COMPONENTS OF THE MICROSCOPE

Let us firstly distinguish the optical part of the microscope from the stage. The optical part comprises all of the optical components and the metal parts, which hold them together as illustrated in Figure 8. The stage includes the pedestal, the column and the focusing system (figure 5).

In the construction of this microscope, we recommend that you progress from the bottom upward, just as you would do when building a house. At first, start with the pedestal, then the column, followed by the focusing device and finally the optical part.

SELECTING THE BINOCULARS

If you do not have not any binoculars, you will need to obtain two sets. You can find low cost binoculars in second hand shops, flea markets, and bazaars or at weekend garage sales. Firstly you will need a set of 8 x 30 binoculars, where 8 is the magnification and 30 is the diameter in mm of the objectives. This type of binoculars is widely available. This pair of binoculars will not be altered and will not be permanently fixed to the microscope, so you can continue to use them as a regular pair of binoculars. When buying your binoculars, be sure that they have a good chromatic correction. They should not produce double or blurred images, as occurs when the images are not perfectly superimposed. Keep in mind that this binoculars should have as a wide field of view as possible. This makes viewing more comfortable thereby enhancing the spectacular three-dimensional effect of stereoscopic vision.

The set of binoculars that are to be dismantled must have an objective of 50 mm in diameter in order to handle the displacement of the optical paths (Fig. 4 and 9). Their power is not important since the eyepieces will not used. These binoculars can be 7 x 50, 8 x 50, 10 x 50 or a 16 x 50, etc. These types of binoculars typically cost about 70 to 150 US$ for a new pair, but used pairs can often be purchased for much less. It is important to verify that the objectives have metal rather than plastic locknuts as they will later be used to connect the objective to the microscope.

When selecting this pair of binoculars, it is important to verify their quality. It is usually sufficient just to observe the amount of the chromatic aberration. To do this, you need to point the binoculars towards a TV antennae or tree branches which adequate backlighting or with the sky as a background. Good quality optics will not produce any orange or blue colors along the edges of the antennae or branches, rather the image should appear to be dark and sharp.


MATERIALS

Note:
- all dimensions are in mm
- # refers to thickness
- M refers to the metric screws system
- means diameter

OPTICAL SECTION

- black anodized aluminum square tube # 2 x 45 x 45 x 170 (prisms-housing tube)
- aluminum square tube # 2 x 45 x 45 x 140 (linkage tube)
- black anodized aluminum "U" shaped bar # 2 x 50 x 10 x 170 used to support the binoculars
(These three forms can be usually obtained from companies which produce or install aluminum windows and door frames)
- Plexiglas or rigid black plastic plate # 8 x 30 x 166 (prism-holder plate)
- 3 small black plastic plates # 2 x 41 x 41 (plugs for prism-holder tube and linkage tube)
- stainless steel sheet # 1 x 65 x 105 (support for second objective )
- drawn metal rod 12 x 36 (support for the second objective)
- 1 cylindrical head screw M 3 x 8 (for the prisms-holder plate)
- 2 conical head screws M 3 x 5 (for the "U" shaped bar)
- 4 conical head screws M 2 x 5 (to fix the first objective)
- 2 cap socket head screws M 4 x 7 (for the mounting of the second objective)
- 4 prisms and two 50 mm-diameter objectives. You can scavenge these from an old set of binoculars.


FOCUSING SYSTEM

- rack: pitch=1, 15 x 15 x 275
- pinion: pitch=1 z=15 (z=number of teeth)
- drawn steel rod 6 x 80 (pinion shaft)
- 2 aluminum plates #5 x 40 x 40 (wings)
- steel plate #5 x 50 x 80 (rack baseplate)
- steel square tube #2 x 25 x 25 x 100 (focusing system carriage)
- "L" shaped aluminum bar #2 x 15 x 15 x 100 (focusing system carriage)
- Teflon or Nylon sheet #1 x 43 x 98 (focusing system carriage)
- 6 cap socket head screws M 4 x 7 (for wings)
- 4 flat tip set screws M 4 x 8 (to push against the "L" shaped bar)
- 1 spring pin 2 x 12 (to fix the pinion on the shaft)
- 2 knobs 50 about (for the focusing system movement)
- 2 flat tip set screws M 4 x 8 (for knobs)
- 4 cap socket head screws M 4 x 5 (to fix the linkage tube)


PEDESTAL

- 1 black covered chipboard panel or wood board #15 x180 x 200
- white plastic trim h=15
- contact cement to fix the plastic trimming in place around the base of the pedestal
- 4 hexagonal head screws M 5 x 35 + 4 washer 5 + 4 nuts M5 (for baseplate)
- 4 white rubber feet 20 x 10
- 4 self-tapping screws 3.5 x 20 + 4 washer 4 (for the rubber feet)


OTHER

- epoxy cement to fix prisms in place
- mat black aerosol paint can to coat the inner parts of the optical section
- wood board for the carrying case
- vinyl glue for the wood case
- sheet of methacrylate #3 for the bell


CONSTRUCTION OF THE STAGE
PEDESTAL

You can make the pedestal as shown in Figure 5 using a black Formica-covered chipboard panel 15-mm thick and with sides of 180 x 200 mm. Round off the four corners and apply a white plastic trimming around the board of the panel. Instead of the chipboard panel, you can also use a wood board of the same dimensions. Place four white rubber pads at the four corners on the underside of the pedestal.

Stereoscopic microscope

RACK

Sometimes one can find such rack and pinion mechanisms in shops selling used photographic equipment. Look for old tripods, photographic enlargers or supports for bellows that have simple rack and pinion devices. Otherwise buy a rack of pitch=1 mm and with a cross-section of about 15 x 15 mm. Pay attention that its sides be smooth and devoid of rust, dents and other defects as the carriage of the focusing system slides along these surfaces. Cut a crop in the rack 275 mm long. If necessary, trim the surfaces with a file and sandpaper. The rack also serves to support column. Weld a steel base plate at the bottom to connect it to the pedestal. When you mount the rack on the pedestal, be sure to check that it is at a right angle with a square. If necessary, you can place shims under the base-plate to ensure that the column is perpendicular to the base.


FOCUSING SYSTEM

Two parts form the focusing system: the carriage and the movement device. As detailed in Figure 7, the carriage allows displacements in the vertical direction. The knobs of the movement device allow you to precisely adjust the displacement of the optical section. The coupling of the carriage to the rack ensure that the movement of the carriage is vertical and is kept in alignment with the object. As illustrated in Figure 7, because the square tube of the focusing system fits closely over the rack, it is forced to follow a straight path up and down the rack.

But how can the tube be made to firmly contact rack? This is accomplished by mean of an L-shaped metal bar which sits in the space between the inner wall of the square tube and smooth sides of the rack. A light pressure is applied to the bar by means of four setscrews on adjacent sides of the tube. To give the device a smooth movement, a piece of Teflon or a Nylon sheet 0.5 or 1 mm in thickness is inserted along the three smooth faces of the central rack. You should be able to find commercially available racks with a very good surface finishing which will provide a very regular and smooth movement of the mechanism. Note that because Teflon sheeting should not be folded around a sharp edge, you will have to bevel the edges of the rack as shown in Figure 7.

At this point, the carriage slides along the rack and the fine movement needed for adjusting the focus is provided by the pinion which engages in the rack. As depicted in Figure 7, you will have to cut an opening in the middle of the carriage tube to allow the pinion to engage with the rack.

On two opposite sides of the carriage tube, fix two plates through which the shaft of the pinion shaft passes. To drill the holes for the shaft, you can use the pinion as a driving bearing to guide the drill bit. When you are done, fix the pinion to the shaft with a spring pin 2 x 12.


LINKAGE TUBE

A square aluminum tube is used to link the optical part to the focusing carriage and space them (see Figures 5 and 8). The optical part is kept higher. This allows you to use a shorter column.


FABRICATION OF THE OPTICAL SECTION
DESCRIPTION

The optical section, illustrated in Figure 8, consists of a square aluminum tube. This tube houses the four prisms that are simply glued to a rigid plastic plate. This plate is then connected to the prism housing tube with one screw. An "U" shaped aluminum bar is fastened over the prism housing with 2 screws. This provides a support and guide for the proximal binoculars. The common objective is fixed to the underside of the prism housing. This lens remains in the tube of the dismantled binoculars. At the back of the prism housing tube, there are two threaded holes to affix it to the rack mechanism.


MAKING PIECES OF THE OPTICAL SECTION

To allow light from the common objective to reach the binoculars, you must cut slots into the prism housing and on the "U" shaped bar. As illustrated in Figures 8 and 9, the central slot is cut into the bottom part of the prism housing tube. Figure 9 is a plan view showing the diameter of the objective, the two central prisms and four light beams crossing them. The two innermost light beams correspond to their position when the binoculars are adjusted at the minimal interpupillary distance, whereas the two outer beams correspond to the maximum interpupillary distance.

Imagine collecting light through the objective of the microscope as two beams of 12 mm in diameter (we will explain later where this value is derived). Due to the dimension of the bevels in the prisms used in this design, the light beams cannot have a clearance any smaller than 16 mm. Given that the objective has a diameter of 50 mm, the greater maximum clearance of the two beams is 50-12 = 38 mm. From these values, the maximum displacement of the beams is 38-16 = 22 mm. This range therefore allows you to adjust the interpupillary distance from 50 to about 76-mm.

Based on the above calculations, we deduce that all of the slots should be 12 mm in width, however, it is necessary to make them 2 mm larger, therefore 14 mm wide. In fact, what determine the diameter of the light beams passing through the microscope are two diaphragms of 12 mm in diameter which we will insert in front to both the objectives of the proximal binoculars as will be explained later. The lower central slot should be as long as the diameter of the objective. In this case it is 50 mm.

Figure 11 shows that the distance between two adjacent beams of light as they pass through the prisms. This then allows us to determine that the upper slots will be 7 + 11 + 7 = 25 mm in width.
What remains to be determined is the distance between the two upper slots. To accomplish this, adjust the proximal binoculars to their maximum interpupillary distance and measure the clearance between their objectives (iobmax). Figure 10 shows how to measure the distance between the objectives and the eyepieces of a pair of binoculars.

As depicted in Figure 11, the outer extents of the slots have to be at a distance equal to iobmax + 12 mm. For example, in the case of my binoculars iobmax = 124 mm, the separation distance between the two slots is therefore 124 + 14 = 138 mm. To ensure proper alignment of the slots on the upper "U" shaped bar and the corresponding slots on the top of the prism housing, is advantageous to fasten these two pieces before cutting the slots.

To install the objective, you can use four small screws under the front metal ring of the objective, as shown Figure 8. Note that the objective lenses in a pair binoculars are designed to focus incoming parallel light rays to converging light on the other side. The objective lens of your microscope must have the opposite orientation as in the pair of binoculars; otherwise the image will not form correctly. This means that the surface of the objective lens that was facing the observer in the binoculars must be oriented instead toward the sample as depicted in Figure 2. To do this, you can simply leave the objective in its tube and mount it as shown in Figure 8.

Figures 3 and 8 show a mask with two openings inserted between the objective and the central prisms. In addition to serving as a dust protector, this mask is used to reduce parasitic light that would otherwise reduce the contrast of the image. You can simply cut this mask from a black card. The dimensions of the openings can be obtained from Figure 9.

 

When you observe an object with the microscope, the points that are out of focus are not circular, but longer in the horizontal axis. This is due to the shape of the slots, which are not circular, but rectangular. To correct this problem, you need to insert a circular aperture in front to both the objectives of the proximal binoculars as shown in Figure 12. What essentially determine the diameter of the light beams passing through the microscope are these diaphragms. Their diameter should be 12 mm. Why 12 mm? As shown Figure 9, an aperture of this diameter allows light to enter the optical section but prevents any reflection against the borders of the prisms and of the objective, when the beams are at their minimum or maximum distance. However, if you do not expect to use the proximal binoculars at their minimum or maximum interpupillary distance, you can make these diaphragms 14 mm in diameter. These apertures can be made by simply cutting a hole in a disk made of black cardboard. You can also use the two lens covers of your binoculars and used a socket punch to perforate the holes.

Cut the slots two mm larger in diameter than the apertures, thus14 mm (or 16 mm). The front mask placed between the objective lens and the prisms, however, must have the openings no greater than 12 mm (or 14 mm) in diameter to minimize parasitic light.


MOUNTING THE PRISMS

Before you start cementing the prisms to the base plate, you must first complete all of the essential components of the microscope. In fact, to complete the adjustment of the prisms, the mechanical part of the instrument should be fully functional.

To assemble the prisms, use a rigid base plate made of a material like Plexiglas or Bakelite. Its thickness has to be such that the prisms will align symmetrically under the slots. The thickness of the base plate is not critical because of the width of the slots is smaller than that of the prisms. On the base plate, trace a centerline that will serve as a reference line to position the prisms. Before you cement the prisms, you will need to make a threaded hole to fasten the base plate. Be sure to carefully clean the prisms.

Next you will have to cement the internal prisms to the base plate. You will have to mount them nearly close to one another as shown in Figure 11. Place a few drops of a two-component epoxy resin on the appropriate faces of the two prisms to be cemented. Following the arrangement shown in Figure 14, place the prisms in the right position referring to the centerline you traced on the base plate. To prevent scratching of the central prisms during their assembly inside the square tube, keep them a few tenths of a millimeter above the plate. This can be done during gluing operation by inserting strips of paper between prisms and supporting surface.

Depending on the curing time of the epoxy cement, the resin should have hardened enough to prevent any movement of prisms under its own weight but it should be sufficiently soft to allow the prisms to be adjusted by applying a little force. At this point, you have to check and adjust the alignment of the prisms to ensure they are co-planar. If their bottom surfaces are coplanar, the reflected images will coincide perfectly as if it was a single mirror. Wait until the epoxy has properly set before proceeding any further with the assembly.

From Figure 11, you can easily obtain the mounting position of external prisms. The "d" dimension defines their position, which is the distance from the diagonals of a couple of prisms, measured following the horizontal direction. It corresponds also to the horizontal tract of light paths. Calculating this value is simple, and you can refer to the maximum objective clearance of the proximal binoculars (iobmax):

d = (iobmax-38)/2

For example, if iobmax is 124 mm, then d = (124-38)/2 = 43 mm.

As light crossing the prisms is parallel, the "d" dimension is not critical. You can verify this distance by measuring it with a ruler. In any case, try to reduce the error to a minimum.

To glue the external prisms, first place the base plate on a table. Secondly, place the epoxy resin on the base plate and then the prisms on the plate. With a ruler verify the position of the prisms and correct it if needed. For this operation, you can also use a plastic or wood shim. To align the prisms to the upper edge of the plate, you can use a metal guide.


ALIGNMENT OF THE PRISMS

The difficulty in setting the prisms comes mainly from the fact that a small departure from the correct position produces an important misalignment of the two images. Even if you carefully mount the prisms, the images produced will not overlap perfectly. To ensure the the prisms are well aligned, adjust them until the images are correctly superimposed. You must complete this operation within few minutes, before the epoxy resin sets or solidifies.

The errors in the positioning of the prisms are of two types: displacements in the XYZ directions and rotation around the three spatial axes. Avoiding rotational error is most critical for ensuring that superimposed images are formed, so try to be as accurate as possible when positioning the prisms. If the mounting of the prisms is done with reasonable care, the remaining alignment errors should be quite small. To correct them, it should be sufficient to adjust only one of the prisms.

The setting of the alignment of the prisms is tricky and requires care and patience. To facilitate this operation, we have divided it up into several steps. Before proceeding, we need to define a few terms. Horizontal alignment is the relative displacements of the images along the horizontal direction (<--->), vertical alignment indicates relative displacement along the vertical direction (v ^), angular alignment means errors in the parallelism of the images ( \ / ).

A) Alignment of couples of prisms. Make this adjustment few minutes after gluing the external prisms. Start with the pair of prisms on the right side. Referring to Figure 15, look at a distant object (about 20 m). If you use a more distant object, you will have an important horizontal misalignment between the images. In any case, during this adjustment, do not pay too much attention to the horizontal alignment because it will be corrected in a later step, but carefully correct any vertical misalignment. If the pair of prisms is well aligned, the image seen through the prism and the one seen directly, are continuous. If needed, correct the position of the external prism. Make the same with the pair of prisms on the left side. Finally, verify that a horizontal line, appearing through the two central prisms is at the same height. If necessary, you may have to raise one corner of the prisms slightly. Make these fine adjustments by inserting thin wood wedges under prisms. This will prevent the elastic recovery of the epoxy resin from moving the prism out of alignment.

Before continuing, let everything rest while the epoxy resin begins to cure while the plate is held in the vertical position. This will preventing the prisms from moving under their own weight however it will still be possible to move them by applying a slightly greater force.

B) Alignment of the prisms as a whole. Now that you have adjusted each pair of prisms independently of one another, we can now proceed to adjust the alignment of the prisms as a whole. Firstly, mount the base plate with the prisms in its square tube. Secondly, put the 8 x 30 binoculars on the microscope and look at a some fine print. This is the "word-test": it is a severe test of the alignment. Make the needed corrections always using wooden wedges (Fig. 16). In this step, the adjustments should be done only for the horizontal alignment of the prisms, but it will be necessary to correct any other misalignments you are able to detect. Moreover, our eyes have a tendency to compensate for alignment errors, mainly in the horizontal direction.

During the final adjustments, I found it useful to mask one eye with a card, leave rest it for a wile, and then quickly uncover it. In this way, I could better detect whether the images were well aligned or not. To help the users of your microscope not to have to cross their eyes too much, you may need to adjust the horizontal alignment. To do so, look now and then, at an object that is about a meter away then look immediately into the microscope while trying to maintain roughly the same convergence of your eyes. If necessary, rotate one of the prisms until the horizontal misalignment disappears. In these steps, you will have to be patient but do not despair, you will see that your microscope will yield well-aligned images.

When you have finished with these adjustments, lay down the microscope in such a way that the prisms exert their weight on the plate. Let the parts rest for about 30 minutes. Check to see if the prisms have maintained their correct alignment and wait a couple more hours for the epoxy resin to properly harden, at which time the microscope will be ready to use. The following day, remove the base plate and remove the wedges under the prisms or cut them with a sharp blade.


FINISHING
PROTECTION AGAINST PARASITIC LIGHT

If you are near a window or lamp, stray light can enter from the openings at each end of the tube used to house the prisms. This will cause images to lose some contrast. To overcome this problem, you need cover the ends of the tube with plugs. Otherwise, the plugs on the linkage tube have only an aesthetic function. These plugs are simply inserted into the tube with light pressure.


BLACKENING THE INSIDE PARTS

Before the final assembly of the microscope, you should blacken the inner surfaces of the tubes used to hold the prisms, the sides of the slots and the inner surfaces of the linkage tube. You can do this quite easily by applying a mat black spray paint. Remember to cover the surfaces you want to protect from the paint with paper and adhesive tape. Allow the painted parts to dry for 24 hours before reassembling the microscope.


FIXING THE BINOCULARS

Finally to view the image, the proximal binoculars are simply placed on top of the microscope. In this position, however, the binoculars could fall and should be fixed to the mechanism. This can be accomplished by means of a fork-shaped support.


MAGNIFYING POWER AND ITS VARIATION

The magnification power of this stereoscopic microscope is given by:

Im = 250 x In/Fd

where:
Im = magnification of the microscope
In = nominal magnification of the proximal binoculars
Fd = focal length of the common objective

If as proximal binoculars you have an "8 x 30", and if Fd is 200 mm, then:

Im = (250 x 8)/200

Im = 10 X

To determine the focal length of your lens, place the lens between a lamp and a screen then adjust the position of the lamp to get a sharply focussed image on the screen. Measure the distance A from the lens center to the lamp and the distance B from the lens center to the screen. The focal length is given by:

F = AxB/(A+B)

From the F value, you have to subtract the distance of the nodes of the lens. You can consider this value to be roughly one half of the lens thickness (I hope that no opticians read this!).

Many users will surely like having more than one magnification, just as commercial microscopes usually do. To double the magnification of the microscope, you can add the other objective lens of the binoculars that you dismantled for this project. The performance of the microscope with double magnification is good, although it is not as sharp as when a single objective is used. You can mount the second objective on a rotating support (Fig. 17). Note that the tube of the first objective has to be cut in order to maintain the shortest distance possible between the two lenses.

To change the magnification, you could also replace the eyepiece with more powerful ones. Another method would be to use proximal binoculars with a zoom feature, but these are often costly and the quality can be poor.

I examined the possibility of using zoom photographic objectives to make a zoom stereo microscope. Unfortunately, these objectives have a small working aperture, which hinders the possibility of adjusting the interpupillary distance. To use such zoom lenses would require changing the structure of the microscope. Work on this project is in progress.


SURFACE FINISHING OF PIECES

When you choose the materials, try to avoid metals that are sensitive to corrosion. You should use nickel-plated steel for the square tube of the focusing system. This galvanic treatment performed on the rack would make it finer, but the differences in the thickness of nickel layers would hinder the movement of the pinion. For this reason, do not plate the rack nor the pinion and its shaft. Moreover the parts of the focusing system should be made of quenched and tempered steel and they are quite resistant rusting as long as you keep the microscope indoors. Aluminum surfaces are also good untreated, but they can also be black anodized. This treatment is suitable for used aluminum parts that are bought and show signs of wear. Consult your local "yellow pages" or business directory for companies that can to the anodizing for you.


CASE

It is springtime, and it's time to get outdoors for a nature excursion! You cannot very well bring your instrument with you as it is. It has been sitting at home all the winter and now it is time to work those lenses and grease up the focusing gears. As it is a precious and fragile instrument, it must be protected. One way of protecting your microscope is to house it in a wooden case. The case can also be used to store the components of the microscope including a light source and accessories that can be placed in special drawers or compartments. The case can be made of plywood or any suitable wood. A wooden case with tongue and groove joints is delightful to see. A nicely finished case will give your microscope a certain prestige and esthetic appeal.


DUST COVER

The microscope that you have built with your own hands is something in which you can take pride. But if you leave it in its case, nobody will ever notice it. On the other hand, keeping it on your desk or workbench will expose it to dust. To protect your microscope, you can cover it with a plastic hood but this does not exhibit your microscope very well and does not protect well against dust.

A good solution is to make a bell cover made of Plexiglas. This is a commonly available transparent and rigid plastic material that you can find also in a variety of colors including smoky gray. To make a bell cover, use a 3-mm thick sheet of this material. As illustrated in Figure 18, you have to bend it in two positions. To do this, you take advantage its thermoplastic properties. This material softens at about 100 C. In order to bend it, use a metal bar heated in an oven. Apply the heated bar along the bending line on one side of the plastic sheet and then to the other side. To prevent scratching of the surfaces, place a thin sheet of paper or a thin cotton tissue between the plastic sheet and the heated bar. When the plastic softens, remove the bar and fold it. When you have got your correct angles, you have to cut two pieces of Plexiglas to close the sides of the bell. Glue them with chloroform, which is a solvent for this plastic, or use an adhesive for methacrylate. This Plexiglas bell case is a simple and elegant solution. It shows the microscope while protecting it from dust.


NAMEPLATE

Below the microscope or on a side of the tube used to house the prisms, you can place a little plate with your name and the date of construction. Consult you local business directory for companies that make or engrave nameplates.


USE OF THE MICROSCOPE
ADJUSTMENTS

The set screws which push the "L" shape of the focusing device have to be tightened slightly: just enough to stop the focusing carriage dropping by itself. To prevent squeaking of the pinion shaft, put a few of oil into the holes through which it passes.

Put the proximal binoculars in a symmetric position by over the slots. If they are not parallel to the "U" shaped form, you will tend to see double images in the vertical direction. This can be useful also to adjust for any slight errors in the alignment of the prisms.


SUPPORTING PLANE

The microscope is quite high with the binoculars in the vertical disposition. If you place the microscope on a normal table, you will have to stretch your neck or do you observations from a standing position. Unfortunately, it can be tiring maintain this posture for a long time. A possible solution is to place the microscope on a low table or a small bench or stool. If you have a suitable table, you greatly increase your comfort when observing for prolonged periods of time.


ILLUMINATION

When you are outdoors, you can use the direct sunlight. But, if you want to see fine details, you have to diffuse this light by means of a suitable translucent or reflecting white screen. When you are at home, you can use the microscope with a table lamp, but you can obtain better results with a powerful directional light. With a good illumination, you will see a fine play of colors and shades that enhance the relief and the colors of the specimens. For this, you can use a 20-Watt halogen lamp. Choose a model that you can easily orient. Unfortunately, these lamps produce heat, which is fatal to many insects, since these creature are accustomed living in a cool and moist environment. With a strong light, rich in infrared, live specimens are at risk of dying because of desiccation under prolonged exposure the strong light and heat. In these cases, you should to use this type of light for only short periods of time then release the specimen. You can filter the light using a heat absorbing filter that lowers the transmission of the hottest frequencies of light and gives a more cold light. You can find such filters in slide projectors or in a shop.


ACCESSORIES

The stereo microscope is a research instrument. When using the microscope, you may need to manipulate the samples you observe. Your use of the microscope can be made easier if you have some of the following accessories:

- a few petri dishes to contain liquids or insects for viewing
- a pair of tweezers with thin tip
- a black card on which to put samples and displace them easily
- plastic jars or vials with screw cap to collect samples of water from ponds
- a glass Pasteur pipette
- transparent boxes to collect insects
- a box to collect vegetables, lichens and mushrooms to avoid crush them
- a bag to collect humus
- a heat absorbing glass for the spot light
- a camera adapter for taking photographs through the microscope
- a daylight filter (to take pictures at artificial light)
- screwdriver and spanners to adjust the microscope


OBSERVATIONS

Great! You have finished your instrument. Finally you can use it and enjoy the results of your hard work. The quality of the images produced by your microscope will be of excellent quality and comparable to images produced by instruments available on the market, which are priced at a thousand dollars or more.

As for the choice of objects to observe, there would be too many things to say for this document. I have found that insects, flowers, minerals, are inexhaustible sources of amazement. I have noticed that the microscope reveals things that are often completely unexpected. Because of this, it is a good idea to invite some friends or family members to share your adventure. It is often quite astounding to see the way in which nature has adapted to resolve certain problems. Consider for example a little yellow spider of a flower. As all arachnids, it does not have compound eyes like insects. Each of its eight eyes is made up of a hemispherical lens. But these eyes do not rotate like ours. Then, how does a spider direct its sight? If you observe closely behind the two central eyes, which are the most important ones, you may see two dark colored shapes moving from side to side. These are the retinas!

You will rarely find something that is not worth viewing. One of your fingers, especially in summer, may show little beads of sweat, which twinkle under the light then to dry up as the water evaporates. A rose bud infested with aphids is a spectacular sight showing winged individuals, adults, pupa, whelping females and molting. If you are patient in your observations, you can also watch a mosquito hatch from the pupa stage. During winter, a snowfall provides ample opportunities to admire the wonderful structure of snow crystals.

Take some frog eggs for example. If you are lucky enough to collect them just after they are laid, you can view the first phases of the egg cell division (Fig. 19). In fact, the fertilized cells divide first in two parts, then four, eight, and so on until the cells become so small that you cannot see them even with the stereoscopic microscope. In the blood vessels of tadpoles you can see the movement of the blood cells. The passage of the blood in the branchiae of a newt is really quite spectacular.

Figure 19. Cell division in a frog's egg.

The study of fresh water aquatic life can be fascinating especially when viewed under the stereoscopic microscope. In pond water, you can see a variety of insect larvae, little crustaceans, small colonies as volvox and vorticella, protists, etc. Some protists are large enough to be observed even with low magnification. The shapes of rotifers, paramecium, diatoms and the way they swim are fascinating. Foraminifers are microscopic shell creatures that live in both fresh water environments like ponds, lakes and rivers and brackish waters like marine environments. Foraminifers have also been around for a long time and fossilized foraminifers are even used by paleontologists to determine the relative age of sedimentary rock formations and to determine the environmental conditions in the distant past. In figure 20, you can see some fossil foraminifera collected from a deposit of Pliocene clay in Val di Zena, near Pianoro (Bologna), Italy.

Try putting a snail on a sheet of glass and observe it. When it opens its pulmonary orifice to breathe, you can also see some of its internal organs. An ant is very fine specimen to view, but it runs around tirelessly and is not easy to follow with the microscope. To get the ant to stay still, place a drop of water sweetened with honey in a petri dish. Gently capture the ant and put it in the petri dish. As soon as it finds the honey water, it should stop to drink, which will allow you to examine it. A caterpillar nibbling on a leaf is a lot of fun to observe.

Figure 20: Foraminifera as seen under the stereoscopic microscope.

Once, when I was observing a caterpillar, I noticed some black grains on the table. After a while, these grains increased in number. I did not understand what was happening, until, at a lower magnification, I saw the caterpillar raise the posterior part of its body to shoot small pill-shaped excrement. In a pond of water, you can find many life forms to observe with your microscope. If you go into wooded area, pick up a few of soil samples: you will be able see many small insects, most of them of primitive species. A mushroom quickly becomes the prey of several parasites. Even when it is fresh, among its gills, you can find numerous small insects feeding on the spores. What about the wonderful structure of butterfly wings with all its little colored tiles? For a mineral collection, you need only obtain small mineral samples. When viewed under the microscopes these samples will show many perfect crystals of different all shapes and colors which often intersect each other.


PHOTOGRAPHS

Let us mention briefly another aspect of this project, which would be the object of a book in itself. With this microscope, you can also take pictures. Its vertical layout makes it ideal for mounting a camera. You can use the camera either with or without objective. In both cases, adjust focus with the microscope. If the field you see is small and circular, adjust the vertical position of the camera so that the pupil of the eyepiece is in the same plane as the camera's diaphragm. When you use the camera without an objective, you can obtain a larger magnification simply by moving the camera away from the eyepiece. In both cases, you need to make an adapter to center your camera on the eyepiece of the microscope to prevent parasitic light from entering the camera. If your adapter is linked to a bellows, you can easily adjust the magnification. To determine the right exposure, your camera should have a TTL exposure; otherwise you will need to calibrate your exposures by doing tests. To do time lapse photography you will have to use a flash unit. You can also link a television camera to the microscope and record to videotape. The methods to setup the camera are the same as for a regular camera. You can record interesting events on videotape, or simply connect it to a video monitor or television to show your family and friends what is happening under the microscope.


MAINTENANCE

You should keep the microscope in a wooden case or under a dust cover to protect it from the dust. If you see rust on any steel pieces, rub the surfaces with an oily cloth slightly. From time to time you may need to tighten some of the screws. If you have to clean optical surfaces, use a clean cotton cloth or optical cleaning paper. Before doing this, remove the dust with a brush, because grains of sand can scratch surfaces of the lenses. In any case, do this operation sparingly. In fact what affect microscopes are not so much dust particles, but films of material which accumulate on the lenses after a prolonged exposure to the ambient air. To prevent this, avoid smoking near the microscope.


CONCLUSION

Until now, the construction of a stereoscopic microscope was beyond the reach of the amateur naturalist. Through an innovative optical approach, the mechanical structure of the microscope is simplified making its fabrication accessible to anyone without the need for specialized machining tools. The construction of this microscope is beneficial from a educational point of view since it provides the young scientist an opportunity to learn some of the basic principles of optics, it also requires one to work out the mechanical details of the project, to obtain the necessary components and to assemble the microscope. A hand made instrument is something in which one can take pride in having.

For parents, this project provides an opportunity share in the learning experience with their sons. This microscope can be a means to encourage the discovery of natural sciences including botany, ecology, entomology, geology, mineralogy and paleontology. While this instrument can be a precious tool for the amateur naturalist, what makes the microscope such a fantastic instrument is your curiosity and ability to be amazed by the microscopic world around us. Without this curiosity, your instrument will be destined to collect dust.


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