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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.

 

src=”rhen_02.jpg” width=”300″ height=”450″>

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.
� of the object
field = 0.36 mm. � 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.20×25, pp.120.

 


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