144 



DRA WING WITH THE MICROSCOPE 



[CH V 



tance of 250 mm. at which the drawing surface should be placed when deter- 

 mining magnification {\ 178). 



§ 202. *Abbe Camera Lucida. — This consists of a cube of 

 glass cut into two triangular prisms and silvered on the cut surface 

 of the upper one. A small oval hole is then cut out of the center of 

 the silvered surface and the two prisms are cemented together in the 

 form of the original cube with a perforated 45 degree mirror within 

 it (Fig. 124, a b). The upper surface of the cube is covered by a 

 perforated metal plate. This cube is placed over the ocular in such 

 a way that the light from the microscope passes through the hole in 

 the silvered face and thence directly to the eye. Light from the 

 drawing surface is reflected by a mirror to the silvered surface of 

 the prism and reflected by this surface to the eye in company with 

 the rays from the microscope, so that the two fields appear as one, 

 and the image is seen as if on the drawing surface (Figs. 124, 129). 

 It is designed for use with a vertical microscope. [Compare § 205.] 



Fig. 127. Wollaston's Cam- 

 era Lucida, showing the rays 

 from the microscope and from the 

 drawing surface, and the position 

 of the pupil of the eye. See also 

 Fig. 112. 



For full explanation see Fig. 108 



*For some persons the image and drawing surf ace.pencil, etc. , do not appear 

 on the drawing board as stated above, but under the microscope, according 

 to the general principle that "objects appear in space where they could be 

 touched along a perpendicular to the retinal surface stimulated," — that is in 

 the line of rays entering the eye. This is always the case with the Wollaston 

 camera lucida. The explanation of the apparent location of the image, etc., 

 on the drawing board with the Abbe camera lucida is that the attention is con- 

 centrated upon the drawing surface rather than upon the object under the 

 microscope (Dr. W. B. Pillsbury) . 



