218 



Binoculars for the Highest Powers. 



prisms, which gave a definition that has never been surpassed by 

 any succeeding arrangements. This was abandoned in favour of 

 the present well-known form, as it required a separate pair of 

 bodies with which direct single vision could not be obtained. But 

 as for some special observations separate bodies are still used, I 

 have re-arranged the prism to suit the angle required. If the 

 vision is taken off at right angles to the axis of object-glass by 

 prisms, in the manner first suggested by me in a paper read before 

 the Microscopical Society in May, 1853, and figured in the ' Trans- 

 actions,' the vision will be erect in one plane. Such was the case 

 in the instrument then described, but the effect was pseudoscopic. 



The following diagram, Fig. 1, shows an enlarged sectional 

 view of the achromatic separating prism now used. The lower 



prism is of crown glass. Fig. 2 (full size) represents its appli- 

 cation to a right-angled or diagonal erecting microscope with 

 horizontal stage. 



The reflecting prisms at a are the same as formerly described, 

 and are set so that the faces are perpendicular to the emergent ray. 

 Some play should be left between them, to allow for lateral adjust- 



