WENHAM S STEREOSCOPIC BINOCULAR. 



33 



WeDham's Prism. 



placed in the tube which, carries the objective (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 principal 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 sub- FlG - 19 - 



jected to any refraction, since 

 its axial ray is perpendicular to 

 the surface it meets ; within the 

 prism it is subjected to two re- 

 flexions at b and c, which send 

 it forth again obliquely in the 

 line d towards the eye -piece of 

 the secondary body l ; and since 

 at its emergence its axial ray is 

 again perpendicular to the sur- 

 face 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 which have come on 

 without any disturbance whatever ; whilst the image received by the 

 leftEjeis formed by the rays which have passed through the right half 

 of the Objective, and which have been subjected to two reflexions 

 within the prism, passing through only two surfaces of glass. The 

 adjustment for the variation of distance between the axes of the 

 Eyes in different individuals, is made by drawing-out or pushing-in 

 the Eye-pieces, which are moved consentaneously by means of a 

 milled-head, as shown in Fig. 21. — Now although it may be 

 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 picture formed by them will 

 be somewhat larger than that which is formed by the other set ; and 

 (2) that the picture 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, — 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, pro- 

 vided that the outlines of the latter are sufficiently distinct to 

 represent its perspective projection. Hence if, instead of the two 



