OPTICAL PRINCIPLES OF THE MICROSCOPE. 29 



ral triangle; and is divided by reflection within this prism into two lat- 

 eral halves, which cross each other in its interior. The rays a b that 

 form the right half of the cone, impinging very obliquely on the internal 

 face of the prism, suffer total reflection ( 2), emerging through its left 

 side perpendicularly to its surface, and therefore undergoing no refrac- 

 tion; whilst the rays a' b' forming the left half of the cone, are reflected 

 in like manner towards the right. Each of these pencils is received bv 

 .a lateral prism, which again changes its direction, so as to render it par- 

 allel to its original course; and thus the two halves a b and a' b f of the 

 -original pencil are completely separated from each other, the former 

 being received into the left-hand body of the Microscope (Fig. -18), and 

 the latter into its right-hand body. These two bodies are parallel; and, 

 by means of an adjusting screw at their base, which alters the distance 

 between the central and the lateral prisms, they can be separated from 

 or approximated towards each other, so that the distance between their 

 -axes can be brought into exact coincidence with the distance between the 

 axes of the eyes of the individual observer. This instrument gives true 

 Stereoscopic projection to the conjoint image formed by the mental fu- 

 sion of the two distinct pictures; and with low powers of moderate angu- 

 lar aperture its performance is highly satisfactory. There are, however, 

 certain drawbacks to its general utility. First, every ray of each pencil 

 suffers two reflections, and has to pass" through /wr surfaces; this neces- 

 sarily involves a considerable loss of light, with a further liability to the 

 impairment of the image by the smallest want of exactness in the form 

 of either of the prisms. Second, the mechanical arrangements requisite 

 for varying the distance of the bodies, involve an additional liability to 

 derangement in the adjustment of the prisms. Third, the instrument 

 can only be used for its own special purpose; so that the observer must 

 also be provided with an ordinary single-bodied Microscope, for the ex- 

 amination of objects unsuited to the powers of his Binocular. Fourth, 

 the parallelism of the bodies involves parallelism of the axes of the ob- 

 server's eyes, the maintenance of which for any length of time is fatiguing. 

 34. Wenham's Stereoscopic Binocular. All these objections are over- 

 come in the admirable arrangement devised by the ingenuity of Mr. 

 Wenham; in whose Binocular the cone of rays proceeding upwards from 

 the objective is divided by the interposition of a prism of the peculiar 

 form shown in Fig. 19, so placed in the tube which carries the objec- 

 tive (Figs. 20, 21 a) as only to interrupt one half, a c, of the cone, 

 the other half, a b, going on continuously to the eye-piece of the prin- 

 cipal or right-hand body R, in the axis of which the objective is placed. 

 The interrupted half of the cone (Fig. 19, a), on its entrance into the 

 prism, is scarcely subjected to any refraction, since its axial ray is per- 

 pendicular to the surface it meets; but within the prism it is subjected 

 to two reflections at b and c, which send it forth again obliquely in the 

 line d towards the eye-piece of the secondary, or left hand body (Fig. 20, 

 L); and since at its emergence its axial ray is again perpendicular 

 to the surface of the glass, it suffers no more refraction on passing out 

 of the prism than on entering it. By this arrangement, the image 

 received by the right eye is formed by the rays which have passed through 

 the left half of the objective, and have come on without any interruption 

 whatever; whilst the image received by the left eye is formed by the 

 rays which have passed through the right half of the objective, and have 

 been subjected to two reflections within the prism, passing through only 

 two surfaces of glass. The adjustment for the variation of distance 



