Microscopy

“ Smaller and eminently suitable for the general worker is “Modern Microscopy,” by Messrs Cross & Cole. Those who wish to specialize in one of the sciences, such as Botany, Zoology, or Geology, will find no lack of books dealing with the subject that most appeals to them. For photographers who are also microscopists, we know of no better books than “Practical Principles of Plain Photomicrography,” by G. West, or the more advanced “Handbook of Photomicrography,” by Messrs Hind & Randles.
Neither books nor teacher will be able to reveal to any of us all the secrets of the microscope. ”

“ In the compound microscope the initial image, formed by the object-glass, is further magnified by another set of lenses, forming the eye-piece, by which the diverging rays of the virtual image are brought together to a focus at the eye-point; and when viewed directly, the eye sees an imaginary image, as in a simple microscope, whilst, when the rays are allowed to fall upon a screen, they form a real image of the object, larger or smaller, as the screen is farther from or nearer to the eye-point. ”

“ For a microscope to be a compound one, its essential qualification is that it should have an object-glass or objective, and an eye-glass or eye-piece, so called because they are respectively near the object and the eye of the observer when the instrument is in use. The microscope consists of a tube or body, and a stand, an arrangement for carrying the body, combined with which is a stage for holding the object, and a mirror for its illumination. To the more modern instrument has been added a substage, to carry a condenser and other accessories. ”

“ The first is that if the light upon the paper be stronger than that in the apparent field of the microscope, the image will not be well seen, or if the paper be too feebly lighted, it will be difficult to keep the point of the pencil in view. The light from the microscope is thrown into the eye, and the view of the image upon the paper is the effect of a mental act, the eye looking out in the direction from which the rays appear to come. ”

“ Opposite to the microscope, and about a foot away from it, is a lamp with the edge of the flame presented to the microscope, the concave mirror of which is so arranged as to receive the rays from the flame and direct them up the tube of the microscope. Upon the stage is placed a piece of ground-glass, and the mirror-arm is now to be moved up or down upon its support until the ground-glass receives the maximum of illumination, which it will do when the lamp-flame is at one conjugate focus of the mirror and the ground-glass at the other. ”

“ Microscopes are either single or double. In the former case, but one lens is used, and through this the object is viewed directly; but, in the latter case, two or more glasses are employed, through one of which a magnified image is thrown upon a reflecting surface, and this is viewed through the other glass, or glasses, as the real object is seen through a single microscope.
21. The solar microscope, on account of its great magnifying powers, is the most wonderful instrument of this kind. The principles of its construction are the same with those of the camera obscura. ”

“ Having now explained the optical principles of the achromatic compound microscope, it remains only to describe the mechanical arrangements for giving those principles their full effect. The mechanism of a microscope is of much more importance than might be imagined by those who have not studied the subject. In the first place, steadiness, or freedom from vibration, and most particularly freedom from any vibrations which are not equally communicated to the object under examination, and to the lenses by which it is viewed, is a point of the utmost consequence. ”

“ Even after the improvement of the simple microscope by the use of doublets and triplets, the long course of the rays, and the large angular pencil required in the compound instrument, deterred the most sanguine from anticipating the period when they should be conducted through such a path free both from spherical and chromatic errors. Within twenty years of the present period, philosophers of no less eminence than M. Blot and Dr. Wollaston predicted that the compound would never rival the simple microscope, and that the idea of achromatizing its object-glass was hopeless. ”

“ 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. ”

“ There are three features of this ingenious apparatus which cannot fail to commend themselves equally to the casual worker and the serious microscopist. All objects are seen erect, just as the eye sees them. This is brought about because, as in the case of the super-microscope, we really observe a magnified, inverted image of our object, formed at the spot where the object would be placed were we using an ordinary microscope. ”

“ They are at length received by the eye-lens L M, which acts upon them as the simple microscope has been described to act on the light proceeding from its objects. They are bent so that they may enter the eye at E in parallel lines, or as nearly so as is requisite for distinct vision. When we say that the rays enter the eye in nearly parallel lines, we mean only those which proceed from one point of the original object. Thus the two parallel rays M E have proceeded from and are part of the cone of rays C A D, emanating from the point A of the arrow ”

“ Along with this I purchased every possible accessory,—draw-tubes, micrometers, a camera-lucida, lever-stage, acromatic condensers, white cloud illuminators, prisms, parabolic condensers, polarizing apparatus, forceps, aquatic boxes, fishing-tubes, with a host of other articles, all of which would have been useful in the hands of an experienced microscopist, but, as I afterwards discovered, were not of the slightest present value to me. It takes years of practice to know how to use a complicated microscope. ”

“ Although it is possible to employ compound microscopes of such a high magnifying power in the investigation of certain classes of objects, all ordinary preparations are best seen under a power of 500 or 600 diameters. It would be utterly impossible to give our readers the slightest idea of the benefits conferred on the human race by this marvellous instrument. Suffice it to say, that no naturalist or surgeon ever attempts the most simple investigation into the structure of any body without the aid of the microscope. ”

“ It may be taken as an axiom with microscopists that “neither the penetrating power nor the high-power defining objective is alone sufficient for every kind of work. The larger the details of ultimate structure, the narrower the aperture—and the converse; the minuter the dimensions of elementary structure, the wider must be the aperture of the objective.” Every worker with the microscope must have satisfied himself of the truth of this statement, when engaged in the study of the movements of living organisms, or defining the intimate structure of the minuter diatoms, or of the podura scale. ”

“ Some simple microscopes consist of two or more lenses mounted together in order to increase the magnifying power. The student must distinguish between several lenses mounted together in this way, and the true compound microscope, which is a comparatively complicated optical arrangement, as we shall see presently. When two single lenses are thus mounted together, the power of the combination is equal to the powers of each added together.
There is good reason for supposing that the simple microscope is a comparatively ancient invention. ”

“ Microscopic agency, in thus assisting the medical man, contributes much to the alleviation of those multiplied “ills which flesh is heir to.” So fully impressed were the Council of the Royal College of Surgeons with the importance of the facts brought to light in a short space of time, that, in 1841, they determined to establish a Professorship of Histology, and to form a collection of preparations of the elementary tissues of both animals and vegetables, healthy and morbid, which should illustrate the value of microscopical investigations in physiology and medical science. ”

“ In place of the image being traced by projection on paper, the reverse is the case, both the paper and pencil are projected into the field of view. The mirror reflects the paper on to the silvered surface of a prism placed over the eye-lens of the eye-piece of the microscope, and it is thereby conveyed to the eye. There is a central opening in the silvering through which microscopic vision is obtained. ”

“ To make a microscope such as I shall now describe, requires little mechanical skill. If my directions are followed, and strict attention given to the drawings, no difficulty will be encountered, but neatness and precision are of course essential First provide a wood rod about 15 inches long, and of the circumference of Fig. 3.
Then take some paper of firm texture, and wind it around the rod three or four times according to its thickness, applying mucilage all the time; immediately withdraw the paper casing, and place on one side to dry. This should form a perfectly true and firm tube. ”

“ Take a plate of brass (Fig. 183) three inches by one, perforated with a series of holes from about one-sixteenth to one-fourth of an inch in diameter; the size of the smallest should be in accordance with the power of the objective, and the separating power of the double refraction.
Experiment 1.—Place the brass plate so that the smallest hole shall be in the centre of the stage of the microscope; employ a low power (11⁄2 or 2 inches) objective, and adjust the focus as for the ordinary microscopic object; place the double image prism over the eye-piece, and two distinct images will be seen ”

“ In the ordinary compound microscope, it is only possible for one person to see the object to be examined at once; for popular exhibitions of microscopic objects the reflecting microscope has been devised, by means of which the images of the objects to be looked at are thrown upon a screen. ”

“ By the term simple microscope is meant one in which the object is viewed directly through a lens or combination of lenses, just as we have supposed an arrow or an insect to be viewed through a glass held in the hand. ”

“ It is also necessary that the analyst should be thoroughly acquainted with the application of the microscope, to determine the cellular structure, to make determinations of proximate principles in the substances under examination, since a mechanical separation by the microscope is more expeditious and is more at the command of the majority of persons searching for adulterations. For a mechanical analysis of food separations, a powerful microscope of good workmanship is required. It is better if it is supplied with a substance condenser and Nical prisms for the use of polarized light. ”

“ Any object which is to be viewed under the microscope must, of course, be supported in some way—this is now usually done by placing it upon a glass slide, which on account of the transparency has a great advantage over other substances. These “slides” are almost always made of one size, viz., three inches long by one broad, generally having the edges ground so as to remove all danger of scratching or cutting any object with which they may come in contact. The glass must be very good, else the surface will always2 present the appearance of uncleanliness and dust. ”

“ Having obtained a sharp image we remove the paper and gradually advance our second lens towards the first. We soon reach a point where we have a very much larger image of the candle than the first lens gave us; we [53] must fix our second lens at this point for we now have a compound microscope, a very crude one certainly and without the trimmings which make the microscope so useful. Before we proceed to explain what has happened to the light rays we must take our paper screen once more and place it as near as possible to the spot where our eye was situated when we saw the second image. ”

“ Opaque objects should be examined by reflected light, by means of the bull’s-eye condenser, or side reflector. Mr. Sorby uses a binocular microscope, which enables him to regulate the focussing and throwing out of focus of the object.
In examining crystals or other small objects, a small cardboard diaphragm should be placed beneath them; and when examining the spectra of liquids in cells, slip a small cap with a perforation of 1⁄10-inch in diameter over the tube containing the 1⁄2-inch or 2-inch objective. ”

“ The fundamental basis for the microscopic examination of any food product must depend upon the structure of the material which enters into its composition. Any attempt to determine an abnormal condition, such as decomposition, without a knowledge of the normal, must necessarily be of little value. There is some work which can be done in a mechanical manner by almost anyone capable of looking through a microscope, and if the work is properly supervised, it may have a value, but the lines along which this can be done are very limited. ”

“ All of us are familiar with the appearance of a microscope ready adjusted for use, with its little lamp, its mirror and illuminating apparatus under the stage, with a specimen on the stage under the object-glass, its object-glass and its eye-piece. Any one who understands the working of a microscope finds this a most suggestive spectacle. ”