WENHAM'S BINOCULAR PRLSM 99 



to the surface of the glass, it suffers 110 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 

 ji;i>sed 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 reflexions within 

 the prism, passing through only fcooj&urfhces 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. 75. Now, although it may be objected to Mr. Wenham's method 

 (1) that, as the rays which pass through the prisin and are obliquely 

 reflected into the secondary body traverse a longer distance than 

 those which pass on uninterruptedly into the principal body, the 

 picture formed by them will be somewhat larger than that which 

 is formed by the other set ; but this can be easily compensated for 

 by (a) altering the power of one of the eye-pieces, (b) by increasing the 

 tube length of the direct tube ; 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 differ- 

 ence in the size of the two pictures is no bar to their perfect com- 

 bination ; 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 ; secondly, that this binocular arrangement does not necessi- 

 tate a special instrument, but may be applied to any microscope 

 which is capable of carrying the weight of the secondary body, the 

 prisin 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 thirdly, that the simplicity of its construction renders its de- 

 rangement almost impossible. 1 



1 The Author cannot allow this opportunity to pass without expressing his sense 

 of the liberality with which Mr. Wenham freely presented to the public this im- 



H 2 



