MICROMETRY /IND THE CAMERA LUCIDA. 



55 



posed one upon- the other. Fig. 27 indicates diagram- 

 matically the general arrangement of the mechanism. 

 At P, just over the eyepiece, is a cube made up of 

 two triangular prisms of glass with a silvered surface 

 between them pierced by a central opening. Through 

 this opening the rays from the microscope pass, while 

 those coming from the paper, AB, are reflected first from 

 the swinging mirror at the side and then upward from 



Fig. 26. — Camera Lucida. (After Gage.) 



the silvered surface of the prisms. In order to avoid 

 distortion it is necessary that the mirror should be at an 

 angle of 45" to the surface on which the paper rests. 

 In order to bring this about it is necessary to sup- 

 port the paper on an inchned surface when the base of 

 the microscope extends outward so as to interfere with a 

 view of the table. 



The adjustment of light with the camera lucida is a 

 matter of considerable difficulty. If the light passing 

 through the microscope be too bright,- only the object will 



