37 



PHYSIOLOGICAL PHYSICS. [Ch ap . 



the tube, in the direction P', to the eye of the observer. 

 Rays from the opposite side of the object are reflected 

 from u' into the prism s, and from its face are thrown 

 in the direction p to the other eye of the observer. 



An arrangement of Nachet's, capable of being 

 adapted to any microscope, is represented in Fig. 

 165. Above the objective a is a totally reflecting prism 

 D, so placed as to receive half of the rays 

 from the object c passing through a. 

 The rays are reflected by D into a second 

 prism E, by which they are again re- 

 flected, and pass up the microscope tube 

 to the eye-piece A'B'. The other half of 

 the rays pursue their straight course 

 unmolested to the eye-piece AB. The 

 prisms can be arranged so as to permit 

 the rays from the right half of the 

 objective to reach the right eye, and 

 the rays from the left half the left 

 eye, and so produce a pseudoscopic 

 effect ; or they may be arranged to 

 cross the rays 

 scopic picture. 



of Nachet's the additional tube can be removed with 

 its prisms, and the microscope used as an ordinary 

 monocular instrument. 



In Wenham's arrangement a single prism of pe- 

 culiar shape, placed above the objective, effects the 

 same purpose as the two of Nachet's. Hartnack has 

 contrived a binocular eye-piece in which there are 

 four rectangular prisms (Fig. 166) placed as shown. 

 Rays proceeding up the tube of the microscope 

 towards E are intercepted by the prisms A and B, 

 and totally reflected. Half proceed towards D, 

 where they are again reflected up into the ob- 

 server's eye at F, while the other half proceed 

 towards c, and are reflected up to the other eye at E. 



Ar- 



rangeruent for 

 converting a 



Monocular into 



, 



arranged 



and ' ive the true stereo- 

 With this arrangement 



