MICROMETRY AND 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 
Fic. 26.—Camera Lucrpa. (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 inclined 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 
