IOO 



VISION WITH THE COMPOUND MICROSCOPE 



Flu. 70. Riddell's binocular 

 prisms, as applied by Mr. 

 Stephenson (1870). 



Stephenson's Binocular. A new form of stereoscopic binocular 

 ha-, been introduced l>v -Mr. Stephenson, 1 which has certain dis- 

 tinctive features, :iii<l ;it 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 Ricldell'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- 

 M-ription and completion of his binocular. 2 

 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 tw r o prisms. 

 and its two halves are similarly acted on, 

 the two pictures are equally illuminated, 

 and of the same size ; while 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 two 

 bodies can be made to converge at a 

 smaller angle than in the Wenham ar- 

 rangement, the observer looks through 

 ' hem with more comfort. But Mr. Ste- 

 phen M in'singeniousarrangementisliable 

 to the great drawback of not being 

 convertible (like Mr. Wenham's) into 

 an ordinary monocular by the with- 

 draual of a prism, so that the use of 

 this form of it \\ill be probably re- 

 stricted to those who desire to work 

 with a binocular when employing high 

 powers. 



I '>ut one of the greatest advantages 

 attendant on Mr. Stephenson's con- 

 struction is iK capability of lieing combined with an erectiny 

 which renders it applicable to purposes for which 

 """ binocular cannol be conveniently used. |',\- the in- 

 '' I'l'ine silvered mirror, or (still better) of a reflecting 



| can !). no doubt, be might have lur-vly pro- 

 lusi 



I rol. vii.(l!S7'2), P. 107. 

 l 1. 



Fio. 77. StepliriiM.n's i'ivctiii<_' 

 i --70). 



