Part 12

The telescope is one of those which are composed of several sliding drawers or tubes, for the convenience of being put into the pocket; the sliding tubes are made of thin brass, the outside tube of mahogany. The sliding tubes are contrived to stop, when drawn out to a proper length, so that by applying one hand to the outside tube, and the other hand to the end of the smallest tube, the telescope may at one pull be drawn out to its full length; then any of the tubes (that next the eye is most generally used) may be pushed in gradually, while you are looking through it, till the object is rendered distinct to the eye. To make the tubes slide properly, they all pass through short springs or tubes; these springs may be unscrewed from the ends of the sliding tubes, by means of the milled edges which project above the tubes, and the tubes taken from each other if required, and the springs set closer if at any time they be too weak.

Fig. 5 represents the exterior tube of the telescope, which is to be unscrewed from the rest, at _m l_, as it does not make any part of the microscope; the cover, _k_, which protects the object-glass, serves also as a box to contain two ivory wheels, Fig. 1 and 2, with the objects, and a small mirror, Fig. 6.

Fig. 4 is a view of this cover when taken off: unscrew the top part of it, and the mirror, Fig. 6, may be taken out; unscrew the cover of the lower part, and you will find therein the two circular object-wheels above mentioned.

Fig. 3 represents the three internal tubes of the telescope, which constitute the microscopic part thereof. Draw the tubes out in the manner as shewn in the figure; then at the inside, but at the lower end of the exterior tube, a, you will find a short tube, which serves as a stage to hold the object and support the mirror; pull this tube partly out, and turn it, so that a circular hole which is pierced in it may coincide with a similar hole in the exterior tube. This tube is represented as drawn out at Fig. 3, the mirror, Fig. 6, placed therein at _b c_, and the transparent object-wheel fixed at a.

Fig. 1 represents the slider with transparent objects.

Fig. 2, that with the opake. They are made of ivory, and turn on a pin at the center; the slit end of this pin fits on the edge of the tube, which is then to be pushed up, so that the lower end of the exterior tube may bear lightly on the upper side of the slider, agreeable to the view which is given at a, Fig. 3. Now push down the second tube till the milled part falls on the milled edge of the extreme tube, being careful of the circular hole in the exterior one. Nothing now remains to be done but to adjust for the focus, which is effected by pushing in the tube R, and moving only the first, _n_.

The instrument may be used in two ways for transparent objects: first, in a vertical position, when the light is to be thrown on the object by the mirror, _b c_; or it may be examined by looking up directly at the light; in the latter case the mirror must be taken away. In viewing opake objects the mirror is not used; as much light as possible must be admitted on them through the circular holes of the tubes. Any object may be viewed by first pushing in the tube, R, and then bringing the tube, _n_, to its focal distance from the object. The telescope, when shut up, is about eight inches in length, and when drawn out, is about twenty inches. It is of the achromatic construction.

DESCRIPTION OF AN INSTRUMENT FOR CUTTING THIN TRANSVERSE SECTIONS OF WOOD, Plate IX. Fig. 1.

It consists of a wooden base, which supports four brass pillars; on the top of the pillars is placed a flat piece of brass, near the middle of which there is a triangular hole. A sharp knife which moves in a diagonal direction, is fixed on the upper side of the afore-mentioned plate, and in such a manner, that the edge always coincides with the surface thereof. The knife is moved backwards and forwards by means of the handle, _a_. The piece of wood is placed in the triangular trough, which is under the brass plate, and is to be kept steady therein by a milled screw which is fitted to the trough; the wood is to be pressed forward for cutting, by the micrometer screw, _b_. The pieces of wood should be applied to this instrument immediately on being taken out of the ground, or else they should be soaked for some time in water, to soften them, so that they may not hurt the edge of the knife. When the edge of the knife is brought in contact with the piece of wood, a small quantity of spirit of wine should be poured on the surface of the wood, to prevent its curling up; it will also make it adhere to the knife, from which it may be removed by pressing a piece of blotting paper on it.

Fig. 2, is an appendage to the cutting engine, which may be used instead of the micrometer screw, being by some practitioners preferred to it. It is placed over the triangular hole, and kept flat down upon the surface of the brass plate, while the piece of wood is pressed against a circular piece of brass which is on the under side of it. This circular piece of brass is fixed to a screw, by which its distance from the flat plate on which the knife moves may be regulated.[38]

[38] Many other kinds of cutting engines have been constructed, but the specimens from them have not yet appeared with that perfection which is requisite to this sort of objects; whether it lies in the preparation of the woods, or engine, I do not take upon me to determine. Mr. Custance has certainly produced the most exquisite. EDIT.

CHAP. IV.

GENERAL INSTRUCTIONS FOR USING THE MICROSCOPE AND PREPARING THE OBJECTS.

As the advantages which are obtained from any instrument are considerably increased, if it be used by a person who is master of its properties, attentive to its adjustments, and habituated by practice to the minutiæ of management, it is the design of this chapter to point out those circumstances which more peculiarly require the attention of the observer, and to give such plain directions, as may enable him to examine any object with ease; to shew how he may place it in the best point of view, and if necessary, prepare it for observation.

A small degree of diligence will render the observer master of every necessary rule, and a little practice will make them familiar and habitual: the pains he takes to acquire these habits will be rewarded by an increasing attachment to his instrument, and the wonders it displays. Let him only persevere till he has overcome the natural indolence that opposes the advancement of every kind of knowledge, and he will most assuredly find himself very amply recompensed, by the gratification arising from the acquisition of a science that has the unlimited treasures of INFINITE WISDOM for the object of its researches: and his mind being strengthened by the victory it has gained, will be more keen in perceiving, and more patient in the investigation of truth.

It has long been a complaint,[39] that many of those who purchase microscopes are so little acquainted with their general and extensive usefulness, and so much at a loss for objects to examine by them, that after diverting themselves and their friends some few times with what they find in the sliders, which generally accompany the instrument, or perhaps two or three common objects, the microscope is laid aside as of little further value: whereas no instrument has yet appeared in the world capable of affording so constant, various, and satisfactory an entertainment to the mind. This complaint will, I hope, be obviated by these Essays, in which I have endeavoured to make the use of the microscope easy, point out an immense variety of objects, and direct the observer how to prepare them for examination.

[39] Baker’s Microscope made Easy, p. 51.

The subject treated of in this chapter naturally divides itself into three heads: the first describes the necessary preparation and adjustment of the microscope; the second treats of the proper quantity of the light, and the best method of adapting it to the objects under examination; and the third shews how to prepare and preserve the various objects, that their nature, organization, and texture, may be properly understood.

OF THE NECESSARY PREPARATION OF THE MICROSCOPE FOR OBSERVATION.

We have in the last chapter explained those particulars that constitute the difference of one microscope from another, and shewn the manner of using each instrument, and how the several parts are to be applied to it. We shall now proceed to give some general directions applicable to every microscope. The observer is therefore supposed to have made himself master of his instrument, and to know how to adapt the different parts of the apparatus to their proper places.

The first circumstance necessary to be examined into, is, whether the different glasses belonging to the microscope are perfectly clean or not; if they be not clean, they must be taken out and wiped with a piece of wash leather, taking care at the same time not to soil the surface of the glass with the fingers: in replacing the glasses, you must also be careful not to lay them in an oblique situation, to place the convex sides as before, and if one glass be taken out, wiped, and replaced before the next, it may prevent the misplacing of them by an unskilful hand.

The object should be brought as near the center of the field of view as possible, for there only will it be exhibited in the greatest perfection.

The eye should be moved up and down from the eye-glass of a compound microscope, till you find that situation where the largest field, and most distinct view of the object is obtained; and as the sight differs very much in different persons, and even in the same person, we frequently find each eye to have a different sight from the other, particularly in those called myopes, or short-sighted, every one ought to adjust the microscope to his own eye, and not depend upon the situation in which it was placed by another.

Care must be taken not to let the breath fall upon the eye-glass, nor to hold that part of the body of the microscope where the glasses are placed with a warm hand, because the damp that is expelled from the metal by the heat will be attracted and condensed by the glasses, and obstruct the sight of the object.

The observer should always begin with a small magnifying power; with this he will gain an accurate idea of the situation and connection of the whole, and will therefore be less liable to form any erroneous opinion, when the parts are viewed separately by a deeper lens. By a shallow magnifier he will also discover those parts which merit a further investigation. Objects that are transparent will bear a much greater magnifying power than those that are opake.

Every object should, if possible, be examined first in that position which is most natural to it: if this circumstance be neglected, very inadequate ideas of the structure of the whole, as well as of the connection and use of the parts, will be formed. If it be a living animal, care must be taken not to squeeze, hurt, or discompose it.

There is a great difference between merely viewing an object by the microscope, and investigating its nature: in the first, we only consider the magnified representation thereof; in the second, we endeavour to analyse and discover its nature and relation to other objects. In the first case, we receive the impression of an image formed by the action of the glasses; in the second, we form our judgment by investigating this image. It is easy to view the image which is offered to the eye, but not so easy to form a judgment of the things that are seen; an extensive knowledge of the subject, great patience, and many experiments, will be found necessary for this purpose: for there are many circumstances where the images seen may be very similar, though originating from substances totally different; it is here the penetration of the observer will be exercised, to discover the difference, and avoid error.[40]

[40] Fontana sur les Poisons, vol. ii, p. 245.

Hence Mr. Baker cautions us against forming too suddenly an opinion of any microscopic object, and not to draw our inferences till after repeated experiments and examinations of the objects, in all lights and various positions; to pass no judgment upon things extended by force, or contracted by dryness, or in any manner out of a natural state, without making suitable allowances.

The true colour of objects cannot be properly determined when viewed through the deepest magnifiers; for, as the pores and interstices of an object are enlarged, according to the magnifying power of the glasses made use of, the component parts of its substance will appear separated many thousand times farther asunder than they do to the naked eye; it is, therefore, very probable, that the reflection of the light from these particles will be very different, and exhibit different colours.

Some consideration is also necessary in forming a judgment of the motion of living creatures, or even of fluids, when seen through the microscope; for as the moving body, and the space wherein it moves, are magnified, the motion will also be increased.

If an object be so opake as not to suffer any light to pass through it, as much as possible must be thrown on its upper surface, by that part of the apparatus which is peculiarly adapted to opake objects. As the apertures of deep magnifiers are but small, and consequently admit but little light, they are not proper for the examination of opake objects: this, however, naturally leads us to our second head.

OF THE MANAGEMENT OF THE LIGHT.

The pleasure arising from a just view of a microscopic object, the distinctness of vision, &c. depend on a due management of the light, and adapting the quantity of it to the nature of the object, and the focus of the magnifier; therefore, an object should always be viewed in various degrees of light. It is difficult to distinguish in some objects between a prominency and a depression, between a shadow and a black stain; and in colour, between a reflection and a whiteness; a truth which the reader will find fully exemplified in the examination of the eye of the libellula, and other flies, which will be found to appear exceedingly different in one position of the light from what they do in another.

The brightness of an object depends on the quantity of light; the distinctness of vision, on regulating the quantity to the object; for some will be lost and drowned, as it were, in a quantity of light that is scarce sufficient to render another visible, as a different portion of light under the same apparatus will often exhibit in perfection, or totally conceal an object in the substance to be examined. This is more particularly the case with the animalculæ infusoriæ, whose thin and transparent form blend as it were with the water in which they swim; the degree of light must therefore be suited to the object, which, if dark, will be seen best in a strong and full light, but if very transparent, it should be examined in a fainter.

A strong light may be thrown on an object various ways: first, by means of the sun and a convex lens; for this purpose, place the microscope about three feet from a southern window; take a deep convex lens, that is mounted in a semicircle and fixed on a stand, so that its position may be easily varied; place this lens between the object and the window, so that it may collect a considerable number of the solar rays, and refract them on the object, or the mirror of the microscope. If the light thus collected from the sun be too powerful, it may be tempered by placing a piece of oil paper, or a glass lightly greyed, between the object and the lens: by these means, a convenient degree of light may be obtained, and diffused in an equal manner over the whole surface of an object, a circumstance that should be particularly attended to; for if the light be thrown in an irregular manner, that is, larger portions of it on some parts than on others, it will not be distinctly exhibited.

Where the solar light is preferred, it will be found very convenient to darken the room, and to reflect the rays of the sun on the above mentioned lens, by means of the mirror of a solar microscope fitted to the window-shutter; for, by this apparatus, the observer will be enabled to preserve the light on his object, notwithstanding the motion of the sun.

Cutting off the adventitious light as much as possible, by darkening the room where you are using the microscope, and admitting the light only through a hole in the window-shutter, or at most, keeping one window only open, will also be found very conducive towards producing a distinct view of the object.

As the motion of the sun, and the variable state of our atmosphere, render solar observations both tedious and inconvenient, it will be proper for the observer to be furnished with a large tin lanthorn, made something like the common magic lanthorn, fit to contain one of Argand’s lamps.[41] The lanthorn should have an aperture in front, that may be moved up and down, and capable of holding a lens; by this a pleasing uniform dense light may be easily procured. The lamp should move on a rod, that it may be readily elevated or depressed. The lanthorn may be used for many other purposes, as for viewing of pictures, exhibiting microscopic objects on a screen, &c.

[41] The lamp should not be of the fountain kind, because the rarefaction of the air in the lanthorn will often force the oil over.

Many transparent objects are seen best in a weak light; among these we may place the prepared eyes of flies and animalculæ in fluids; the quantity of light from a lamp or candle may be lessened by removing the microscope to a greater distance from them, or it may be more effectually lessened by cutting off a part of the cone of rays that fall on the object, either by placing the cone, as already described with the apparatus to different microscopes, under the stage, or by forming circular apertures of black paper of different sizes, and placing either a large or small one on the reflecting mirror, as occasion may require.

There is an oblique position of the mirrors, and consequently of the light, which is easily acquired by practice, but for which no general rule can be given, that will exhibit an object more beautifully and more distinctly than any other situation, shewing the surface, as well as those parts through which the light is transmitted.

A better view of most objects is obtained by a candle or lamp than by day-light; it is more easy to modify the former than the latter, and to throw it on the object with different degrees of density. From what has been said, the reader will have observed the importance of being able to examine the object in the greatest variety of positions and appearances, which cannot be effected with equal convenience by any microscope, but the improved lucernal.

OF THE PREPARATION OF OBJECTS FOR THE MICROSCOPE.

In the preparation of objects, no man was more successful or more indefatigable than Swammerdam. In minutely anatomizing, in patiently investigating, and in curiously exhibiting the minute wonders of the creation, he stands unrivalled, far exceeding all those that preceded, as well as those which have succeeded him. Deeply impressed and warmly animated by the amazing scenes that he continually discovered, his zeal in pursuit of truth was not to be abated by disappointment, or alarmed by difficulty; and he was never satisfied till he had attained a rational and clear idea of the organization of the object, whose structure he wished to explore; his “Book of Nature,” of which a translation was published by Dr. Hill, is a work of such vast extent of knowledge, and so excellent in execution, as to raise the highest admiration in even a superficial observer.

It is much to be regretted, that we are ignorant of the methods he employed in his investigations. To discover these, the great Boerhaave examined with a scrupulous attention all the letters and manuscripts of Swammerdam, and has communicated the result of his researches, which, though but small, may enable us to form some idea of his immense labours in the field of science.

For dissecting of small insects he had a brass table, which was made by that excellent artist, S. Musschenbroeck; to this table were affixed two brass arms, moveable at pleasure to any part of it. The upper portion of these arms was constructed so as to have a slow vertical motion, by which means the operator could readily alter their height, as he saw most convenient to his purpose; the office of one of these arms was to hold the minute bodies, and that of the other to apply the lens or microscope.

His microscopes or lenses were of various foci, diameters, and sizes, from the least to the greatest, and the best that could be procured in regard to the exactness of the workmanship, and transparency of the substance. His mode was, to begin his observations with the smallest magnifiers, and from thence proceed by degrees to the greatest. Formed by nature, and habituated by experience, he was so incomparably dexterous in the management of these instruments, that he made every observation subservient to the next, and all tend to confirm each other, and complete the description.

His chief art seems to have been in constructing very fine scissars, and giving them an extreme sharpness: these he made use of to cut very minute objects, because they dissected them equally; whereas knives and lancets, let them be ever so fine and sharp, are apt to disorder delicate substances, as in going through them, they generally draw after and displace some of the filaments. His knives, lancets and styles, were so very fine, that he could not see to sharpen them without the assistance of a magnifying glass; but with them he could dissect the intestines of bees with the same accuracy and distinctness that the most celebrated anatomist does those of large animals. He was particularly expert in the management of small glass tubes, which were no thicker than a bristle, and drawn to a very fine point at one end, but thicker at the other. These he made use of to shew and blow up the smallest vessels discovered by the microscope, to trace, distinguish, and separate their courses and communications, or to inject them with very subtil coloured liquors.

He used to suffocate the insects in spirit of wine, in water, or spirit of turpentine, and likewise preserved them for some time in these liquids; by which means he kept the parts from putrefaction, and consequently from collapsing and mixing together; and added to them besides such strength and firmness, as rendered the dissections more easy and agreeable. When he had divided transversely with his fine scissars the little creature he intended to examine, and had carefully noted every thing that appeared without further dissection, he then proceeded to extract the viscera in a very cautious and deliberate manner, with other instruments of great fineness; first taking care to wash away and separate with very fine pencils, the fat with which insects are very plentifully supplied, and which always prejudices the internal parts before it can be extracted. This operation is best performed upon insects while in the nympha state.