Chapter I: Elementary Optics (3)

Fig. 42 represents a sectional view of Abbe’s stereoscopic eye-pieces, which consist of three prisms of crown glass, _a_, _b_ and _b′_, placed below the field-glass of the two eye-pieces; the tube _c_ is slipped into the tube or body like an ordinary eye-piece. The two prisms _a_ and _b_ are united so as to form a thick plate with parallel sides, inclined to the axis at an angle of 38·5°. The cone of rays from the objective is thus divided into two parts, one being transmitted and the other reflected; that transmitted passing through _a b_ and forming an image of the object in the axial eye-piece B. Adjustment for different distances between the eyes is effected by the screw placed to the right-hand side of the figure, which moves the eye-piece B′, together with the prism _b′_, in a parallel direction. The tubes can also be drawn out, if greater separation is required. The special feature of this instrument is that on halving the cone of rays by turning the caps, an orthoscopic or pseudoscopic effect is produced. This double-eyed piece arrangement of Abbe’s has not been at all brought into use in this country; this is partly owing to its original adaptation for use with the shorter Continental body-tube of 160 mm., and not for our 10-inch body.

The most perfect method of securing pleasing satisfactory stereoscopic vision of objects is that devised by Mr. Wenham. In his binocular microscope an equal division of the cone of rays, after passing through the objective is secured and again united in the eye-pieces, which act as one, so that each eye is furnished with an appropriate and simultaneous view of the object. The methods contrived by the earlier experimenters not only materially interfered with the definition of the objective and object, but also required expensive alterations and adaptations of the microscope, and sometimes separate stands for their employment. Mr. Wenham’s invention, on the contrary, offers no such obstacle to its use, and the utility of the microscope as a _monocular_ is in no way impaired either when using the higher powers.

The most important improvement, then, effected by Wenham consists in the splitting up or dividing the pencil of rays proceeding from the objective by the interposition of a prism of the form shown in Fig. 43. This is placed in the body or tube of the microscope so as to interrupt only one-half (_a c_) of the pencil, the other half (_a b_) proceeding continuously to the field-glass, eye-piece, of the principal body. The interrupted half of the pencil on its entrance into the prism is subjected to very slight refraction, since its axial ray is perpendicular to the surface it meets. Within, the prism is subjected to two reflections at _b_ and _c_, which send it forth again obliquely on the line _b_ towards the eye-piece of the secondary body, to the left-hand side of the figure; and since at its emergence its axial ray is again perpendicular to the surface of the glass, it suffers no further refraction on passing out of the prism than on entering. By this arrangement, the image sent to the right eye is formed by rays which have passed through the left half of the objective; whilst the image sent to the left eye is formed by rays which have passed through the right half, and which have been subjective to two reflections within the prism, and passing through two surfaces of glass. The prism is held by the ends only on the sides of a small brass drawer, so that all the four polished surfaces are accessible, and should slide in so far that its edge may just reach the central line of the objective, and be drawn back against a stop, so as to clear the aperture of the same.

The binocular, then (Fig. 44), consists of a small prism mounted in a brass box A, which slides into an opening immediately above the object-glass, and reflects one-half of the rays which form an image of the object, into an additional tube B, attached at an inclination to the ordinary body C. One half of the rays take the usual course with their performance unaltered; and the remainder, though reflected twice, show no loss of light or definition worthy of notice, if the prism be well made.

As the eyes of different persons are not the same distance apart, the first and most important point to observe in using the binocular is that each eye has a full and clear view of the object. This is easily tried by closing each eye alternately without moving the head, when it may be found that some adjustment is necessary by racking out the draw-tubes D, E, of the bodies by means of the small milled head near the eye-pieces; this will increase the distance of the centres; and, on the contrary, the tubes, when racked down, will suit those eyes that are nearer together.

If the prism be drawn back till stopped by the small milled head, the field of view in the inclined body is darkened, and the rays from the whole aperture of the object-glass pass into the main body as usual, neither the prism nor the additional body interfering in any way with the use of the instrument as a monocular microscope.

The prism can be withdrawn altogether for the purpose of being wiped: this should be done frequently, and very carefully, on all four surfaces, with a perfectly clean cambric or silk handkerchief or a piece of wash-leather; but no hard substance must be used. During this process the small piece of blackened cork fitted between the prism and the thick end of the brass box may be removed; but it must be carefully replaced in the same position, as it serves an important purpose in stopping out extraneous light.

As the binocular microscope gives a real and natural appearance to objects, this effect is considerably increased by employing those kinds of illumination to which the naked eye is accustomed. The most suitable are all the opaque methods where the light is thrown down upon the surface; but for those objects that are semi-transparent, as sections of bone or teeth, diatomaceæ, living aquatic animalcules, &c., the dark-field illumination by means of the parabolic reflector will give an equally good result.

For perfectly transparent illumination, it is much better to diffuse the light by placing under the object various substances, such as tissue-paper, ground glass, very thin porcelain, or a film of yellow bees’ wax, run between two pieces of thin glass.

To ensure the full advantage and relief to both eyes in prolonged observations with high as well as low powers, and with objectives of large aperture, Mr. Wenham devised a compound prism for use with his binocular microscope, the body tubes of which are also made expressly to suit the prism, as extreme accuracy is necessary to bring them into proper position. The main prism somewhat resembles in form the ordinary Wenham prism. Over the first reflecting surface is placed a second smaller prism, the top plane of which is parallel with the base of the first, so that direct rays pass through without deviation, but at the two inclined surfaces of the prisms (nearly in contact) there is a partial reflection from each, which, combined, give as much light as in the direct tube. The reflected image from these two surfaces is directed up into the inclined tube as usual. A somewhat later improvement is that of Dr. Schroeder, the high power prism, by means of which the whole of the rays emanating from the objective pass through it, and the full aperture of any power is thereby effectively utilised. Furthermore, Messrs. Ross have also constructed a right- and left-hand pair of eye-pieces, which ensure greater perfection of the image. It was, in fact, noticed that the size of the image in the left-hand field glass slightly differed from that of the right when examined by the ordinary Huyghenian eye-pieces. To compensate for this difference, the left-hand eye-piece has been carefully calculated, and its focus is now so accurately adjusted that the position of each eye in observing is brought into one plane of the binocular. The pairs of the several series of eye-pieces A, B and C have also been altered, and the effect is to greatly improve the image and give increased comfort to the observer.

Dr. Carpenter, who warmly espoused the binocular, and constantly employed it in his work, very truly said of it: “The important advantages I find it to possess are in penetrating power, or focal depth, which is in every way superior to that of the monocular microscope, so that an object whose surface presents considerable inequalities is very much more distinctly seen with the former than with the latter.”

This difference may in part be attributed to the practical modification in the angle of aperture of the objective, produced by the division of the cone of rays transmitted through the two halves, so that the picture or image received through each half of the objective of 60° is formed by rays diverging at an angle of only about 30°. He confesses, however, that this does not satisfactorily explain the fact that the binocular brings to the _mind’s_ eye the _solid_ image of the object, and thus gives to the observer a good idea of its form and which could hardly be obtained by the monocular microscope. Carpenter cites in support of his views the wing of a little-known moth, _Zenzera Œsculi_, which has an undulating surface, whereon the scales are set at various angles instead of having the usual imbricated arrangement, a good object for demonstrating; the general inequality of surface and the obliquity of its scales, which are at once seen by the binocular with a completeness not obtained by the monocular instrument.

To one unaccustomed to work with the binocular the views expressed by Dr. Carpenter as to the extreme value of the instrument for ordinary work may appear somewhat exaggerated, but from my own experience, having long had in constant use a Ross-Zentmayer binocular, furnished with a special prism, constructed for working with a 1/8 dry objective or a 1/10 immersion, the perfection of picture obtained was in every case quite equal to that of the monocular microscope. The relief to the eyes can hardly be over-estimated; the slight inequality of the pencil rays may be regarded rather as a part of the welcome rest afforded when a prolonged examination is made; it certainly appears to me to equalise the slight physiological difference known to exist between the eyes of most people. If one image is seen a little clearer by the stronger eye, the weaker eye assists rather more the stereoscopic effect of the object under observation. The advantage gained by the binocular is perhaps more appreciated when opaque objects are under examination, as the eggs of insects, and the tongue of the blow-fly, specimens of mosses, lichens, parasites (vegetable and animal), whose planes and inequalities of surface require penetration, and which usually demand more time for their observation.

No variation or change of any kind proposed either in the form of the instrument or the prism has proved of sufficient value or importance to bring it into use, and therefore Wenham’s instrument is scarcely likely to be superseded. It must be admitted that the improvement effected in the eye-piece form by Mr. Tolles, of Boston, U.S., is an exception to the rule laid down. It consists in mounting the prisms in a light material, vulcanite, made to fit into the monocular microscope body, thus taking the place of the ordinary eye-piece. The image transmitted by the objective is brought to a focus on the face of the first equilateral triangular prism by the intervention of an erector-eye-piece inserted beneath it. The second set of prisms have a rack and pinion movement to adjust them to any visual angle. The illumination of both fields in this eye-piece is nearly equal in brightness. Mr. Stephenson’s erecting binocular (Fig. 45) has proved to be of some practical value. It has the advantage of being of equal use with high and low powers, and with little loss of definition. When used for dissecting purposes it gives an erect image of the object. It is equally useful as a working microscope, for arranging diatoms and botanical specimens of every kind. The sub-stage tube will receive a diaphragm or illuminating apparatus; the eye-pieces have a sliding adjustment for regulating the widths between eyes.