OPTICAL PRINCIPLES OF THE MICROSCOPE. 31 



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 derange- 

 ment almost impossible. 1 



35. Stephenson's Binocular. A new form of Stereoscopic Binocular 

 has been recently introduced by Mr. Stephenson, 2 which has certain ad- 

 vantages over both the preceding. The cone of rays passing upwards 

 from the object-glass, meets a pair of prisms (A A, Fig. 22) fixed in the 

 tube of the microscope immediately above the posterior combination of 

 the objective, so as to catch the light-rays on their emergence from it; 

 these it divides into two halves, each of which is subjected to internal 

 reflection from the inner side of the prism 

 through which it passes; and the slight separa- na-22. 



tion of the two prisms at their upper end, gives 

 to the two pencils B B, a divergence which car- 

 ries them through two obliquely-placed bodies 

 to their respective eye-pieces. By this internal 

 reflection, a lateral reversal is produced, which 

 antagonizes the lateral reversal of the Micro- 

 scopic image; so that each eye receives the image 

 formed by its own half of the objective, in the 

 position required for the production of Stereo- 

 scopic relief by the mental combination of the 

 two. As the cone of rays is equally divided by 

 the two prisms, and its two halves are similarly 

 acted-on, the two picture are equally illuminat- 

 ed, and of the same size; while the close ap- 

 proximation of the prisms to the back lens of the 

 objective enables even higher powers to be used 

 with very little loss of light or of definition, 

 provided that the angles and surfaces of the prisms ste P henson ' s Binocul 

 are worked with exactness. And as the two bodies can be made to con- 

 verge at a smaller angle than in the Wenham arrangement, the observer 

 looks through them with more comfort. But Mr. 

 Stephenson's ingenious arrangement which was first Era. -23. 



worked-out practically by the late Thomas Ross, and 

 has since been very successfully constructed by Brown- 

 ing is liable to the great drawback of not being con- 

 vertible (like Mr. Wenham's) into an ordinary Mono- 

 cular, by the withdrawal of a prism; so that the use 

 of this form of it will be probably restricted to those 

 who desire to work stereoscopically with high powers. 

 In order to avoid slight errors arising from the im- 

 pinging of the central ray of the cone, at its emergence 

 from the objective, against the double edge of the 



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- 

 portant inventicm, by which there can be no doubt that he might have largely 

 profited if he had chosen to retain the exclusive right to it. 



2 "Monthly Microscopical Journal," Vol. iv. (1870), p. 61, and Vol. vii. (1872). 

 p. 167. 



