88 



MICROSCOPES FOE SfECIAL rUKPOSES. 



Fig. 4.5. 



Stereoscopic Binocular. This is accomplished by Messrs. Powell 

 and Lealand by taking advantage of the fact already adverted to 

 (§ 1), that when a pencil of rays 

 falls obliquely upon the surface of a 

 refracting medium, a part of it is re- 

 flected without entering that medium 

 at all. In the place usually occupied 

 by the "VYenham prism, they interpose 

 an inclined plate of glass with parallel 

 sides, through which one portion of 

 the rays proceeding upwards from the 

 whole aperture of the Objective passes 

 into the "principal Body with very little 

 change in its course, whilst another 

 portion is reflected from its surface 

 into a rectangular prism so placed to 

 direct it obliquely upwards into the 

 secondary Body (Fig. 45). Although 

 there is a decided difference in bright- 

 ness between the two images,* that 

 formed by the reflected rays being 

 the fainter, yet there is marvellously 

 little loss of definition in either, even 

 when the 25th-inch Objective is used. 

 The disk and prism are fixed in a 

 short tube, which can be readily 

 substituted in any ordinary Binocular 

 Microscope for the one containing the 

 Wenham prism. — The Author can 

 bear the most explicit testimony to 

 the diminution of fatigue resulting 

 from the use of this little apparatus : 

 by which a prolonged employment of 

 high powers is permitted, that would 

 be prejudicial to the eye used singly ; 

 whilst it entirely prevents that bad 

 effect which is liable to proceed from 

 the too exclusive use of a single eye, 

 the impairment of its power of focus- 

 sing consentaneously with the other Powell and Lealand'a Non- 

 eye in ordinary vision. i££££? C BM ™ 



* An arrangement has been devised by Mr Wenham ("Transact, of 

 Microsc. Soc." Vol. xiv. p. 103), by which the brightness of the images 

 is more nearly equalized ; but this involves difficulties of construction 

 with wbich no one save its ingenious inventor has successfully 

 grappled. 



