Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/289

Rh 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 &quot;conversion of relief&quot; produced by this arrangement with the ordinary eye-piece as making a metal spherule appear &quot;as a glass ball silvered on the under side, and a crystal of galena like an empty box.&quot; And to render the images &quot;normal and natural&quot; 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. 1 Subsequently, however, Professor Riddell devised and perfected another arrangement giving a true orthoscopic effect, which, after being long disre garded, has been latterly taken up and brought into use by Mr Stephen- son. The cone of rays passing up wards 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 Fl0 8L -gJJJ S Biuocular at tbeir 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. 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 sur faces 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 afb from the left half of the cone obliquely upwards to the right. These pencils, pass ing 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 sur faces, 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 adjust ing 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 converg ent, 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 on 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 1 See Silliman s Journal, vol. xv., 1853, p. 68 ; and Quart. Jour of Micros Set., vol. i., 1853, p. 236. 2 Quart. Jour, of Micros. Set., vol. ii., 1854. p. IS. FIG. 22. Nachet s Binocular Prisms. 273 to devise a construction by which it might be obtained without the drawbacks inevitable in the workingof Riddell s and Nachet s instru ments; and he soon succeeded in accomplishing this on apian which has proved not only convenient but practically satisfactory, notwith standing its theoretical im perfection. 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 an image that slitters 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 i| in the individual observer) is much more generally suitable than the parallelism of MM. Nachet s earlier instrument ; FIG. 23. Nachet s Binocular Microscope. 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 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 uninter ruptedly 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 phe nomena 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, pro vided that the outlines of the latter are suffi ciently distinct to represent its perspective projection. Hence if, instead of the two equally half-good pictures which are obtain able by MM. Nachet s original construction, we had in Mr Wenham s one good and one FIG. 24. Wenham s indifferent picture, the latter would be de- Stereoscopic Biu- cidedly preferable. But, in point of fact, the ocular Microscope, 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 addi tion, 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 carry ing 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 XVI. - 35