298 



TECHNICAL MICROSCOPY. 



appliances are the most advantageous in which two reflexions- 

 take place in the same plane, e.g., prisms acting by two internal 

 total reflexions. 



' To this category belong Soemmerring's mirror, the older drawing- 

 prism of Nachet, and the equilateral prism all acting by a 

 single reflexion ; and further also the camera lucida of Wollaston, 

 Milne-Edwards, Amici, and Oberhaeuser all acting by two 

 reflexions. The Oberhaeuser camera lucida is provided with a 

 right-angled tube A A (Fig. 166) ; the drawing paper, serving as 

 the plane of projection, may therefore be placed flat upon the 

 work-table, a special stand not being required. The vertical arm 

 of the apparatus is inserted in the body -tube after the eye-piece- 

 has been removed, and the prism C is so adjusted by turning the 

 tube B that its plane of reflexion coincides with that of d. In all 

 the other devices the plane of projection must be so situated that 

 the rays emerging in the direction of the axis of the Microscope 

 meet it perpendicularly after the last reflexion, if produced back- 

 wards ; consequently a special drawing stand must be employed, 

 otherwise the projected image appears distorted. 



(2.) Devices which project the rays of light from the point of 

 the pencil by two reflexions, so that they appear to come from 

 points in the field of view. Here the microscopic image is seen 

 directly through an aperture in the last reflecting surface or 



T7 _.__ near to it ; on the 



other hand, the point 

 of the pencil is seen by 

 ?v reflexion. This prin- 

 '^ ciple may of course be 

 applied with various 

 modifications similar 

 to those of the preced- 

 ing devices. Among 

 the contrivances of 

 this nature we may 

 mention the drawing 

 apparatus of Gerling, 

 the camera lucida of 

 Xachet, Chevalier, Xobert, &c., and Hagenow's Dicatopter (Fig. 

 167). The last is fitted with a horizontal tube, similar to the 

 Oberhaeuser camera, so that the microscopic image undergoes a 



\A 



FlG> 167 



