(E) BINOCULAR MICROSCOPES
Stereoscopic Binoculars.—The admirable invention of the stereoscope by Professor Wheatstone has led to a general appreciation of the value of the conjoint use of both eyes, in conveying to the mind a conception of the solid forms of objects such as the use of either eye singly does not generate with the like certainty or effectiveness (see STEREOSCOPE). This conception is the product of the mental combination of the dissimilar perspective projections which our right and left retinae receive of any object that is sufficiently near the eyes for the formation of two images that are sensibly dissimilar. Now it is obvious that a similar difference must exist between the two perspective projections of any object in relief that are formed by the right and left halves of a microscopic objective and that this difference must increase with the angular aperture of the objective. And the fact of this difference may be easily made apparent experimentally, by adapting a semicircular "stop" to any objective of from 20º to 30º angle in such a manner that it can be turned so as to cover either its right or its left half; for not only will the two images of any projecting object formed by the rays transmitted through the two uncovered halves be found sensibly different, but, if they be photographed or accurately drawn, the "pairing" of their pictures in the, stereoscope will bring out the form of the object in vivid relief. What is needed, therefore to give the true stereoscopic effect to a binocular microscope is a means of so bisecting the colic of rays transmitted by the objective that its two lateral halves shall be transmitted the one to the right and the other to the left eye, and that the two images shall be crossed (the image formed by the right half of the objective being sent to the left eye, and that formed by the left half of the objective being sent to the right eye) in order to neutralize the reversing effect of the microscope itself. If this crossing does not take place, the effect will be rendered "pseudoscopic," not "orthoseopic,"—its projections becoming depressions, and its depressions being brought out as prominences. It was from a want of due appreciation of this fact that the earlier attempts at constructing a stereoscopic binocular gave representations of objects placed under it, not in their true orthoscopic, but in their pseudoscopic aspect. This was the case, for example, with the binocular microscope first devised by Professor Riddell of New Orleans in 1851, which separated the cone of rays by a pair of rectangular prisms so placed edge to edge above the objective that the rays passing through its right half were reflected horizontally to the right side, to be changed to the vertical direction and sent to the right eye by a lateral rectangular prism, while the rays from the left half of the objective were sent to the left eye in a similar manner. Professor Riddell describes the "conversion of relief" produced by this arrangement with the ordinary eye-piece as making a metal spherule appear "as a glass ball silvered on the under side, and a crystal of galena like an empty box." And to render the images "normal and natural" he found himself obliged to use erecting eye-pieces, which should produce a second reversal of the images that had been reversed in their first formation. [Footnote 273-1] Subsequently, however, Professor Riddell devised and perfected another arrangement giving a true orthoscopic, effect, which, after being long disregarded, has been latterly taken up and brought into use by Mr Stephenson. The cone of rays passing upwards from the objective meets a pair of prisms (A, A fig. 21) fixed immediately above its back lens, which divides it into two halves; each of these is subjected to internal reflexion from the inner side of the prism through which it passes; and the slight separation of the two prisms at their upper end gives to the two pencils B,B, on their emergence from the upper surfaces of the prisms, a divergence which directs them through two obliquely-placed bodies to their respective eye-pieces. By this internal reflexion a lateral reversal is produced, which neutralizes that of the ordinary microscopic image, so that, while each eye receives the image formed by its own half of the objective, the pairing ,of the two pictures produces a true orthoscopic effect. [Footnote 273-2]
About the same date MM. Nachet of Paris succeeded in devising a binocular that should give a true orthoscopic image, by placing above the object-glass an equiangular prism (P. fig 22) with one of its surfaces parallel to its back lens, which, receiving the pencils ab forming the right half of the cone, internally reflects them obliquely upwards to the left, and in like manner reflects the pencils a´b´ from the left half of the cone obliquely upwards to the right. These pencils, passing out of the left and right,oblique faces of the prism at right angles (so as not to undergo either refraction or dispersion), enter right and left lateral prisms, also at right angles, and, after being internally reflected by these, pass out vertically, at right angles to their upper surfaces, through two parallel bodies (fig. 23), whose eye-pieces bring them to a focus in the right and left eyes respectively. The distance between these bodies may be adjusted to the varying distances between the axes of individual pairs of eyes, by adjusting screws at their base, which vary the distance of the lateral prisms from the central. This instrument gives a theoretically perfect representation of a microscopic object in relief, as it would appear if enlarged to the size of its image, and brought to within about 10 inches of the eye; and its chief practical defect is that, as the two bodies are parallel, instead of being slightly convergent, it cannot be continuously used without an uncomfortable strain. But, as its performance depends upon the accuracy of the seven plane surfaces of the three prisms, and oil the correctness of their relations to each other, it is liable to considerable error from imperfections in its construction; and, as the instrument can only be used for its own special purpose, the observer must be provided with an ordinary single-bodied microscope for the examination of objects unsuited to the powers of the binocular. This last objection applies also to Professor Riddell’s model.
It was for these reasons that Mr Wenham, fully impressed with the advantages of stereoscopic vision to the microscopist, set himself to devise a construction by which it might be obtained without the Riddell’s and Nachet’s instruments; and he soon succeeded in accomplishing this on a plan which has proved not only convenient but practically satisfactory, notwithstanding its theoretical imperfection. Only the right half of the cone of rays pro-ceeding upwards from the right half of the objective (a, fig. 24) is intercepted by a prism placed immediately over that half of its back lens, which, by two internal re-flexions (as shown in fig. 25), sends its pencils obliquely upwards into the left-hand or secondary body L, whilst the pencils of the left half-cone pass uninterruptedly into the right-hand body R, and form all image that suffers no other deterioration than that which results from the halving of the angular aperture and the con-sequent loss of light. The moderate convergence of the two bodies (which, by varying the angles of the prism, may be made greater or less, so as to accord with the ordinary convergence of the optic axes in the individual observer) is much more generally suitable than the parallelism of MM. Nachet’s earlier instrument; and the adjustment requisite for variation of distance between the eyes can be made by simply lengthening or shortening the bodies by drawing out or pushing in the diverging L eye-pieces.
It may be fairly objected to Mr Wenham’s method (1) that, as the rays which pass through the prism and are obliquely reflected into the secondary body traverse a longer distance than those which pass on uninterruptedly into the principal body, the image formed by them will be somewhat larger than that which is formed by the other set, and (2) that the image formed by the rays which have been subjected to the action of the prism must be inferior in distinctness to that formed by the uninterrupted half of the cone of rays. But these objections are found to have no practical weight. For it is well known to those who have experimented upon the phenomena of stereoscopic vision (1) that a slight difference in the size of the two pictures is no bar to their perfect combination, and (2) that, if one of the pictures be good, the full effect of relief is given to the image, even though the other picture be faint and imperfect, provided that the outlines of the latter are sufficiently distinct to represent its perspective projection. Hence if, instead of the two equally half-good pictures which are obtainable by MM. Nachet’s original construction, we had in Mr Wenham’s one good and one indifferent picture, the latter would be decidedly preferable. But, in point of fact, the deterioration of the second picture in Mr Wenham’s arrangement is less considerable than that of both pictures in the original arrangement of MM Nachet; so that the optical performance of the Wenham binocular is in every way superior. It has, in addition, these further advantages over the preceding:—first, the greater comfort in using it (especially for some length of time together) which results from the convergence of the axes of the eyes at their usual angle for moderately near objects; second, that this binocular arrangement does not necessitate a special instrument, but may be applied to any microscope which is capable of carrying the weight of the secondary body,—the prism being so fixed in a movable frame that it may in a moment be taken out of the tube or replaced therein, so that when it has been removed the principal body acts in every respect as an ordinary microscope, the entire cone of rays passing uninterruptedly into it; and, third, that the simplicity of its construction renders its derangement almost impossible. Hence it is the one most generally preferred by microscopists who use the long-bodied English model.
For short-bodied Continental microscopes, however, MM. Nachet have 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 interesting 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." [Footnote 274-1]
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 microscope. 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 eyepiece 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 eyepieces 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 pseudoscopic conversion. [Footnote 274-2] 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 accommodation possessed by the eye, which, while focussed 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 probable that, as Professor Abbe has urged, this power of accommodation 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 completeness 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 inconsiderable, because the pictures are not sufficiently dissimilar to one another,—the case being exactly analogous to that of the stereoscopic 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 1/2 inches focus and an angular aperture of from 15º to 20º, a very distinct separation is made of the focal planes of transparent sections of structures having no great minuteness of detail,—such, especially, as injected preparations,—the solid forms of 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 20º or 25º is used, the stereoscopic effect becomes much more satisfactory; so that objects of moderate projection (such as many of the siliceous Polycystina, Diatomaceae, &c.) 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 2/3 inch of 30º or 35º, 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 1/2 inch objective of about 40º 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 Dearer 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 4/10ths and 1/4th objectives whose angular aperture does not exceed 50º; but it can be shown both theoretically and practically [Footnote 274-3] that the dissimilarity of the two perspective projections of objects in relief formed by objectives of any angle much exceeding 40º 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 divergence to produce the secondary image.
In regard to the advantage derived from the use of the stereoscopic binocular, with the powers, and upon the objects, suitable to produce the true effect of solid form, the writer can unhesitatingly assert, as the result of a long and varied experience, that in no 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. [Footnote 274-4]
Footnotes
273-1 See Silliman’s Journal, vol. xv., 1853, p. 68; and Quart. Jour. of Micros. Sci., vol. i, 1853, p. 236.
273-2 Quart. Jour. Micros. Sci., vol. ii., 1854, p. 18.
274-1 See Trans. of Roy. Micros. Soc., N. S., vol. xv., 1867, p. 105; and monthly Micros. Jour., vol. i., 1869, p. 31.
274-2 See Jour. of Roy. Micros. Soc., 2d ser., vol. i., 1881, p. 298.
274-3 See The, Microscope and its Revelations, 6th ed., pp. 42-44.
274-4 A very elaborate investigation, by Professor Abbe, "On tile Conditions of Orthoscopic and Pseudoscopic Effects in the Binocular Microscope," will be found in the Jour. of the Roy. Micros. Soc., 2d ser., vol i., 1881. p. 203.
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