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Hooke’s Microscope Revisited

Giorgio Carboni, May 2012
Translated by Sarah Pogue June 2012

 

 

CONTENTS

Introduction
A simple glass-sphere microscope
A compound glass-sphere microscope
Characteristics of 2 glass-sphere objectives
Related microscopes
Conclusion
Bibliography

 

Figure 1 – Optical diagram of Hooke’s microscope.
Particular of diagram 1 of the Micrography.

 

Figure 2 – Hooke’s microscope equipped with modern devices.
 

Figure 3 – Section of a rush stem showing astral cells. Image captured
using the compound glass-sphere microscope described in this article.
Ø of the object field = 0.36 mm. Ø field of vision = 45 mm at a distance
of 25 cm from the eye. Angle of the image  = 10.5 °. Magnification = 147 X.

 

Introduction
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In this article, I will compare two of the first microscopes constructed by man: Robert Hooke’s compound microscope and Antoni van Leeuwenhoek’s simple microscope. In optics, a simple microscope is defined as a microscope formed from a single lens, while a compound microscope is defined as one formed from two lenses or from groups of lenses: the objective and the eyepiece. I am making this comparison because it appears to me that certain very widespread claims regarding van Leeuwenhoek’s microscope are unfounded. During the course of this article, I will also supply the information necessary to construct Hooke’s microscope. By reading this article, and better still by building this microscope, you will gain much useful information about microscopes in general.

A simple glass-sphere microscope
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The ability of glass spheres or of spherical vials filled with water to magnify images has been known since ancient times, however, before the end of the 16th century, nobody had made systematic use of this fact to analyse natural and man-made objects. Antoni van Leeuwenhoek (1632-1723) was the first person to construct and utilise a microscope based on a single small lens. But how did he have the idea to do this? During the course of his life, van Leeuwenhoek dedicated himself to a range of trades including that of cloth merchant. In this field “pearls” of glass were used to value the quality of the cloth. Van Leeuwenhoek realised that the smaller these pearls were, the greater their power of magnification, and he therefore set himself the task of creating very small pearls of glass: as small as 1-2 mm in diameter. During the fabrication of these small spheres or biconvex lenses he used ever finer abrasive powders.

Another method of creating these miniscule spheres was to fuse pieces of glass. At elevated temperatures, glass becomes fluid and the surface tension of the liquid gives the spheres a very precise shape which is maintained when they cool. Handling these tiny spheres was very difficult however, and the instrument that van Leeuwenhoek created served to bring the samples to be observed within a few tenths of a millimetre from the surface of the lens. Despite the fact that van Leeuwenhoek lacked a scientific background, with his microscopes he succeeded in carrying out numerous important observations in the field of microbiology, which he sent to the Royal Society of London.

Unfortunately, it was and still is very difficult to use this microscope. It is with great difficulty that one manages to discern the samples to be observed. In order to facilitate the use of this microscope, in the 1950s Roger Hayward [4, 5] designed a version that enabled the use of microscope slides and was equipped with a mirror to illuminate objects in diascopy. However, to discern the sample to be observed it was still necessary to be very close to the sphere, focussing the sample was very problematic, and the field of vision was also limited. To allow us to distinguish this microscope from the others, let’s call this model "Hayward’s microscope".

Also for the purpose of facilitating the use of this instrument based on a single small lens, a few years ago I equipped it with a lighting system composed of an electric torch and I improved the focussing mechanism. Given the tendency of electric torch bulbs to blow and given the inclination of the batteries to die when they are needed, I recently equipped this microscope with a LED. Therefore, the glass-sphere microscope that I have described in the article http://www.funsci.com/fun3_it/sfera/sfera.htm is descended from that of Hayward and before that from that of van Leeuwenhoek. This is a fascinating instrument because of its simplicity and the results that it gives. Furthermore, it can easily be constructed in your own home using simple tools. There remain, however, a number of problems which we will see how to tackle further on. To distinguish this microscope from other models, we will call it the “FSG glass-sphere microscope".


A compound glass-sphere microscope, Hooke’s microscope  
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It appears that the inventors of the telescope were Hans Janssen and his son Zacharias in the year 1590 (the inventor of this instrument is still under discussion). As it is derived from the telescope, the microscope was born as a compound instrument. In 1665, Robert Hooke fine-tuned a compound microscope equipped with a course and fine focus. Van Leeuwenhoek was born in 1632 and when he began making his observations with his microscope in 1673, there were already various models of compound microscope in existence.

During the same period, Hooke utilised a compound microscope which used a spherical, hemispherical or biconvex lens as its objective and a positive lens as its eyepiece (figure 1). In 1670, Huygens developed an eyepiece free of lateral chromatic aberrations that he had designed for telescopes. The use of this eyepiece in microscopes revealed itself to be very interesting: the distance of the eye from the eyepiece could be great enough to allow comfortable and prolonged observations. The field was sufficiently wide and defined by a precise circle. This microscope had, however, the defect of magnifying the sample too much and therefore producing unclear images with poor contrast. For this microscope, it is preferable to use a hemispherical lens for the objective (it has half the magnifying power of a spherical lens of the same diameter) and an eyepiece with a low magnification (4 X or less).

As I have said, this microscope (figures 1 and 2) reduces the principal inconveniences of the glass-sphere microscope and I have described its main characteristics. I will not propose its construction to you as it is rather laborious. Obviously, I do not discourage you from constructing it either and those who enjoy constructing instruments will certainly take pleasure from this project.
The Huygens’ eyepiece does not utilise glasses of different dispersion such as flint or crown glass, but obtains all the same a reduction in the principal aberrations using lenses with different curvatures positioned at a critical distance. The matching of lenses with the appropriate characteristics to reduce certain aberrations can be carried out only with compound microscopes and not with simple microscopes as they use only a single lens.

Hooke’s microscope can be reproduced for historic reasons, but also for display purposes. In both cases it is important to remain faithful to the original and to use materials such as cardboard, leather, ivory, wood, vellum etc. In this article, I have instead chosen to use a model very close to the original, but have also adopted some important improvements found in modern instruments, such as course and fine focussing mechanisms, a device for slide movement, a retractable support for the objective, a simpler but effective illumination mechanism and a wider tube to further reduce internal reflections. In figure 1, the objective, in this case a hemisphere, is mounted incorrectly due to the fact that in order to reduce the spherical aberrations the flat surface must face the sample. Ultimately, the optics of Hooke’s microscope consist of an objective composed of a single spherical, hemispherical or biconvex lens. The first models used a plano-convex lens as an eyepiece and later on a low-magnification Huygens eyepiece.

As you can see in figure 2, the base and upright of the microscope are made of wood. The macrometric focussing mechanism is fixed to the upright and is composed of two cylindrical corrected chrome guides on which the carriage slides by means of three elastic teflon plain bearings. The carriage is moved by means of a rack and pinion (figure 3). On one of the two tabs that support the manoeuvring bar I have made an incision and mounted a screw which acts as a brake (figure 4), preventing the carriage from falling due to the force of gravity. On the carriage a principal tube is mounted (figure 2) by means of two V-shaped supports. Under the principal tube I have applied a spherical objective, mounted on a retractable support equipped with a spring (figure 5). I have prepared a second objective as a reserve, this is also retractable, but returns to its position due to gravity. The interior of the principal tube is blackened and it is necessary to eliminate any remaining reflection. The specimen stage is fixed to the upright using an L-shaped piece of metal (figure 6). On top of the stage a mechanical slide mover is fixed. Under the stage is the micrometric focussing mechanism created using the differential screw mechanism (figure 6). Also under the stage we find the illumination system. This is based on a white LED and a potentiometer which allows variation of the light intensity. The distance of the LED from the sample is important and should be approximately 20 mm. The use of the eyepiece inevitably transforms the microscope from a simple sphere to a compound sphere instrument: an interesting evolution of van Leeuwenhoek’s microscope.

Should you wish, some of these small spheres can be worked with abrasive powder to obtain convex hemispherical lenses, which are less subject to spherical aberrations. These lenses are mounted with the flat surface facing the sample.

 

Figure 4 – Guide, carriage, rack
and pinion and transverse bar.
Figure 5 - Brake. Note the incision
and the screw on the support tab.

Figure 6 – Glass-sphere objective mounted
at the end of the principal tube.
 
Figure 7 – Micrometric focussing mechanism
and LED illumination system.

The cleaning and quality of the optical components is of great importance, otherwise the image could become less sharp. The out of focus areas of the image (figure 3) correspond to variations in the height of the sample or defects in the optical components. The quality of these components must be verified with a stereoscopic microscope or a strong lens. The glass sphere should be free of bubbles in its interior and must be cleaned with care using water or saliva and a piece of cotton. The spherical surface of the LED should be free of abrasions and must also be cleaned with care. The LED is made of plastic and for this reason it is easy to scratch or mark. The LED fitting must be elastic and must be cleaned with care to avoid damaging the surface of the LED when mounting it. The disc diaphragm is not necessary, and neither is the diffuser. To construct the illumination mechanism, refer to the article on the glass-sphere microscope [1]. The construction of this microscope involved over a month’s work for me.


Characteristics of 2 glass-sphere objectives
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I must explain that van Leeuwenhoek mainly used objectives worked with abrasive powders, and rarely used objectives produced by fusion. In my glass-sphere microscopes, I have only used objectives produced by fusion. For this microscope I have chosen two, one of which has a diameter of 1.60 mm and the other of 1.76 mm. I have subjected these objectives to various tests. During the observation of a Ronchi ruling (composed of alternate opaque and transparent bands) I noted a more than good flatness of the field and only a hint of distorsion, while the microcontrast appeared to be good. The star test exam revealed the presence of some astigmatism and spherical aberration. Probably, the astigmatism derives from the presence of the glass stem which deforms the sphere itself. To avoid this aberration, you can purchase small glass spheres (without a stem). Unfortunately, it is not possible to reduce the spherical aberration. However, at this point, it is possible to obtain from these objectives everything that they have to give.

Related microscopes
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Van Leeuwenhoek’s microscope was so uncomfortable to use that very few people succeeded in doing so. The main problems were due to the lighting system which should have been formed of a uniformly illuminated circular surface. Only those who have attempted to use microscopes of this type know how critical the lighting system is for producing decent images. To facilitate the use of these glass-sphere microscopes, in the 1950s Roger Hayward [4, 5] designed a model equipped with the possibility to use glass microscope slides and an illumination system based on an adjustable mirror (still very ineffective). The instrument that I have presented in this article has the same scope of facilitating the use of glass-sphere objectives. Its structure permits you to obtain, from a miniscule sphere-shaped objective, all that it can give in terms of image sharpness and ease of use, saving the observer a great deal of inconvenience.

Conclusion
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With respect to the simple glass-sphere microscope, this project has introduced an eyepiece that permits better access to the exit pupil with greater comfort for the observer. The instrument is equipped with a simple and effective illumination system. The focussing and slide moving mechanisms also reveal themselves to be convenient and very similar to those of conventional microscopes. This instrument remains largely a stimulus for the studious and those passionate about antique microscopes and in particular glass-sphere microscopes, for whom it will open up new horizons. It will permit them to carry out experiments and comparisons and to better evaluate the capabilities of these microscopes. Finally, it will permit them to realise the importance of cleaning the illumination system and of the integrity of the optical surfaces in obtaining good images from these instruments. This compound microscope, derived from that of Hooke, and the simple microscopes of Hayward or FSG derived from that of van Leeuwenhoek, offer the possibility to use normal microscope slides. It therefore becomes possible to observe the same sample and make more precise comparisons regarding the quality of the image produced by the two different microscopes. The search for the sample is easier and its observation more comfortable. At this point, we have two microscopes, one simple and one compound, which can be considered sufficiently representative from the point of view of optics of the microscopes from which they are derived. We can therefore subject them to comparative examination. It is also possible to use a ruling for the precise evaluation of the instrument characteristics.

 


Figure 7 – Image of the ruling taken with
Hooke’s microscope. 1 division = 2 μm

 
Figure 8 – Image of the ruling taken with the FSG
glass-sphere microscope. 1 division = 2 μm

 

There is a recurring claim that the microscopes produced by van Leeuwenhoek gave superior detail compared to the compound microscopes produced during the same period. As we were not convinced of the accuracy of this statement, we decided to compare the performance of these two microscopes. During an initial comparison, the image produced by the FSG glass-sphere microscope appeared sharper, but was also magnified to a much lesser extent than that produced by Hooke’s microscope, and as we know a lesser degree of magnification is usually accompanied by greater image clarity and contrast. Comparisons are very useful, but until we use quantitative instruments it is difficult to carry out a precise and objective evaluation.

To avoid the influence of the degree of magnification on the image, we have taken a series of photographs of a ruling using both microscopes, we have chosen one for each instrument, attempting to choose the best, and we have magnified them to the same degree. The evaluation of the FSG simple microscope and that of Hooke’s compound microscope, undertaken using a graticule of 100 μm/50 lines (1 div = 2 μm), demonstrated equal capacity: both microscopes, equipped with spherical objectives of the same diameter, resolved the lines of the ruling at a distance of 2 μm one from the other, we could therefore establish that the resolution and the contrast of the images produced by the two microscopes are practically the same (figures 7 and 8).

Without taking photographs and without magnifying them to the same degree, the image of the ruling observed with the FSG microscope appeared sharper, while Hooke’s microscope magnified the image to a greater degree and produced poorly defined images with little contrast. For Hooke’s microscope emerges therefore the utility of using spherical objectives with a greater diameter (e.g. 3 mm) or hemispherical objectives and to employ eyepieces with a low level of magnification to avoid the negative effects of “empty” magnification: lack of clarity and low microcontrast. The resolution of these microscopes is good, particularly if one considers that these are instruments that can be constructed using limited means. The claim of the superiority of the simple microscope with respect to the compound microscope thus appears to be without foundation.

I thank Dr Sini, an expert in microscopy, for his help and for putting the tools necessary for these evaluations at my disposal.

Bibliography
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1 - http://www.funsci.com/fun3_it/sfera/sfera.htm  Glass-sphere microscope.
2 - http://www.funsci.com/fun3_it/sini/mo/microfoto_digitale.pdf 
3 - http://www.funsci.com/fun3_it/pupilla/alta.htm 
4 - C.L. Stong; from: "The Scientific American; Book of projects for the amateur scientist"; 1960; Simon and Schuster Inc. New York
     It is a collection of articles wrote by different authors, devoted to the amateur scientist and edited by C.L. Stong.
5 - Roger Hayward (1899-1979), artist, architect, designer of optical instruments, astronomer.
6 - Turner, Gerard, L'Estrange . Collecting Microscopes. London, A Studio Vista Book published by Cassell Ltd. 1981, cm.20x25, pp.120.

 


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