274 MICROSCOPE FIG. 25. Wenham s Binocular Prism. hare devised an arrangement of two prisms, based on Mr Wenham s fundamental idea of deflecting one half of the cone of rays into a secondary body, whilst the other half proceeds onwards without change of direction into the principal body. And it is an interest ing feature in this construction that, by a simple change in the position of the dividing prism, the true " orthoscopic" image may be made, by a " conversion of relief," to become " pseudoscopic. " l The effect of stereoscopic projection may be attained, however, without a double body, by the insertion of a suitably constructed binocular eye-piece into the body of any ordinary monocular micro scope. A plan of this kind was first successfully worked out by Mr Tolles (the very able optician of Boston, United States), who interposed a system of prisms similar to that devised by MM. Nachet (fig. 22), but on a much larger scale, between an "erector" (resembling that used in the eye piece of a day telescope) and a pair of ordinary Huygenian eye-pieces, the central or dividing prism being placed at or near the plane of the secondary image formed by the erector, while the two eye pieces are placed immediately above the lateral prisms, the combination thus making that division in the pencils forming the secondary (erected) image which it makes in the Nachet binocular in the pencils emerging from the objective. A stereoscopic eye-piece of a very different construction has been recently devised by Professor Abbe, who, making use, for the division between the two eye-pieces of the rays going to form the first image, of an arrangement of prisms essentially similar to that devised by Mr Wenham for his non-stereoscopic binocular (fig. 27), obtains either an orthoscopic or a pseudoscopic effect by placing on each eye-piece a cap with a semicircular diaphragm, so as to extinguish half of each of the cones of rays that form the two retinal images. While in one position of the diaphragms true stereoscopic or orthoscopic relief is given, it is sufficient to turn the diaphragms into the opposite position to obtain a pseudo scopic conversion. 2 It appears, however, that this arrangement, though possessing points of great interest in relation to the theory of binocular vision, is not likely to supersede the ordinary Wenham prism. It must be obvious to every one who studies with sufficient attention the conditions under which true stereoscopic relief can be given that no combination of two dissimilar retinal perspectives can be satisfactory unless the visual pictures represent with tolerable distinctness the features of the object that lie in different focal planes. This is provided for, in ordinary vision, by the power of accommo dation possessed by the eye, which, while focusscd exactly to any one plane, can also include in its visual picture (within certain limits) what is either nearer or more remote. Now it seems prob able that, as Professor Abbe has urged, this power of accommoda tion comes into play in microscopic stereoscopy, but there can be no question that the visual distinctness of the parts of an object lying within and beyond the focal plane, and therefore the com pleteness of the stereoscopic image, mainly depends upon the "focal depth " of the objective employed, which, as already explained, is a function of its angular aperture. When, however, objectives of long focus and small aperture are employed in binocular microscopy, although each of the two perspective projections may be fairly distinct throughout, the effect of solid relief will be very incon siderable, because the pictures are not sufficiently dissimilar to one another, the case being exactly analogous to that of the stereo scopic combination of two photographic portraits taken at an angle of no more than a few degrees from each other. Still, with an objective of 1 inches focus and an angular aperture of from 15 to 20, a very distinct separation is made of the focal planes of trans parent sections of structures having no great minuteness of detail, such, especially, as injected preparations, the solid forms oi their capillary networks being presented to the mind s eye with a vividness that no monocular representation of them can afford. When a 1 inch objective of 20 or 25 is used, the stereoscopic effect becomes much more satisfactory ; so that objects of moderate pro jection (such as many of the siliceous Polycystinn, Dintomaccfe, itc. ) can be seen in nearly their natural projection, and, if the focal adjustment is made for a medium plane, with tolerable distinctness both of their nearer and remoter parts. With a f inch of 30 or
- !5, the stereoscopic relief becomes more pronounced ; but the
diminution of the focal depth prevents the several planes of objects in strong relief from being as distinctly seen at the same time. A See Trans, of Roy. Micros. Soc., N. S., vol. xv., 1867, p. 105; and Monthly Micros. Jour., vol. i., 1869, p. 31. 2 See Jour, of Ro l. Micros. Soc., 2d ser., vol. 1., 1881, p. 298. ^ inch objective of about 40 of aperture, however, affords the most satisfactory results with suitable objects, full stereoscopic relief being gained without exaggeration, so as to present, e.g., the discoidal diatoms and the smaller Polycystina in their true forms, whilst their nearer and more remote parts are seen with sufficient distinctness to require only a very slight adjustment of the focus for their perfect definition. Still more minute objects may be well shown by T Vhs and th objectives whose angular aperture does not exceed 50 ; but it can be shown both theoretically and practically 3 that the dissimilarity of the two perspective projections of objects in relief formed by objectives of any angle much exceed ing 40 is such as to exaggerate the stereoscopic effect ; besides which, every enlargement of angular aperture so greatly diminishes the focal depth of the objectives that only those parts of the objects which lie very near the focal plane can be seen with distinctness sufficient for the formation of a good stereoscopic image. Hence, for the purposes of minute histological research, the stereoscopic binocular is (in the present writer s opinion) almost valueless ; since, if any distinct perspective differentiation can be gained with objectives of the short focus and enlarged angle that are most suitable to such investigations, that differentiation will be so great as to produce a highly exaggerated stereoscopic effect. If such objectives be used binocularly at all, they must be so mounted that their back lenses are in very close proximity to the prism ; and the (transparent) object must be illuminated by an achromatic condenser of sufficient aperture to send through it pencils of sufficient diver gence to produce the secondary image. In regard to the advantage derived from the nse of the stereo scopic binocular, with the powers, and upon the objects, suitable to produce the true effect of solid form, the writer can unhesitat ingly assert, as the result of a long and varied experience, that in IK> other way could he as certainly or as vividly image those forms to himself, and that in prolonged work upon such subjects he is conscious of a great saving of fatigue, which seems attributable not merely (perhaps not so much) to the conjoint use of both eyes as to the absence of the mental effort required for the interpretation of the microscopic picture, when the solid form of the object has to be ideally constructed from it (chiefly by means of the information obtainable through the focal adjustment), instead of being directly presented to the mind s eye. 4 Non- Stereoscopic Binoculars. The great comfort which is experi enced by the microscopist in the conjoint use of both eyes has led to the invention of more than one arrangement by which this can be secured when those high powers are required which cannot be employed with the ordinary stereoscopic binocular. This is accomplished by Messrs Powell and Lea- land by taking advantage of the fact that, when a pencil of rays falls obliquely upon the surface of I a refracting medium, a part of it is reflected with out entering that medium at all. In the place usually occupied by the Wenham prism they interpose an inclined plate of glass with parallel sides, through which one portion of the rays pro ceeding upwards from the whole aperture of the objective passes into the principal body with very little change in its course, whilst another portion is reflected from its surface into a rectangular prism so placed as to direct it obliquely upwards into the secondary body (fig. 26). Although there is a decided difference in brightness between the two images, that formed by the reflected rays being the fainter, yet there is marvellously little loss of definition in either, even when the ^ inch . objective is used. The disk and prism are fixed in a short tube, which can be readily substituted in any ordinary binocular microscope for the one containing the Wenham prism. Other arrangements were devised long ago by Mr Wenham, 5 with a view to obtain a greater equality in the amount of light-rays form ing the two pictures ; and he has latterly carried one of these into practical effect, with tho advantage that the compound prism of which it consists has so nearly the same shape and size as his ordinary stereoscopic prism as to be capable of being mounted in precisely the same manner, so that the one may be readily exchanged for the other. The axial ray a, proceeding upwards from the objective, enters the prism ABDEF (fig. 27) at p . right angles to its lower face, and passes on to c, where it meets the inclined face AB, at which this prism is nearly in contact with the oblique face of the right-angled 3 See The Microscope and its Revelations, Gth cd., pp. 42-44.
- A very elaborate investigation, by Professor Abbe, "On the Conditions of
Orthoscopli: nnd Pseudoscopic Effects in tlie Binocular Microscope," will be found in the Jour, of the Roy. Micros. Soc., 2d ser., vol. i., 1881, p. 203.
9 Transactions of the Micros. Soc., N. S,, vol. xiv., 1866, p. 105.