100 



VISION WITH THE COMPOUND MICKOSCOPE 



tephenson (1870). 



Stephenson's Binocular, A new form of stereoscopic binocular 

 has been introduced by Mr. Stephenson, 1 which has certain dis- 

 tinctive features, and at the time Mr. Stephenson devised it he was 

 entirely unaware that any part of the 

 method he employed had been used by 

 another. He had, however, independently 

 conceived Riddell's device for dividing the 

 beam as a part of his very ingenious in- 

 strument. This he discovered and acknow- 

 ledged about three years after the full de- 

 scription and completion of his binocular. - 

 The cone of rays passing upwards from the 

 object-glass meets a pair of prisms (A A, 

 fig. 76) fixed in the tube of the microscope 

 immediately above the posterior combina- 

 tion of the objective, so as to catch the 

 light-rays on their emergence from it ; 

 these it divides into two halves and be- 

 haves as described in the Riddell prisms, 

 which, in fact, they are. As the cone of 

 rays is equally divided by the two prisms, 

 >> itHtwo halves are similarly acted on, 

 the two pictures are equally illuminated, 

 and of the same size ; w r hile the close ap- 

 proximation of the prisms to the back lens of the objective enables 

 even high powers to be used with very little loss of light or of 

 definition, provided that the angles and surfaces of the prisms are 



worked with exactness ; and as the tw< > 

 bodies can be made to converge at a 

 smaller angle than in the Wenhain ar- 

 rangement, the observer looks through 

 them with more comfort. But Mr. Ste- 

 phenson's ingenious arrangement islial >1 e 

 to the great drawback of not being 

 convertible (like Mr. Wenham's) into 

 an ordinary monocular .by the with- 

 drawal of a prism, so that the use of 

 this form of it will be probably re- 

 stricted to those who desire to work 

 with a binocular when employing high 

 powers. 



But one of the greatest advantages 

 attendant on Mr. Stephenson's con- 



struction is its capability of being combined with an erectiny 

 arrangement, which renders it applicable to purposes for which 

 the Wenhain binocular cannot be conveniently used. By the in- 

 terposition of a plane silvered mirror, or (still better) of a reflecting 



portant invention, by which, there can be no doubt, he might have largely pro- 

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



1 Monthly Microscopical Journal, vol.iv. (1870), p. 61, and vol. vii. (1872), p. 167. 



- Ibid. vol. x. p. 41. 



FIG. 77. Stephenson's erecting 

 prism (1870). 



