together at their diagonal surfaces, thus forming a 

 cube. At one side of the cube about 3 inches from 

 it is a mirror set at an angle of about 45 degrees. 

 The optical parts are mounted in various degrees of 

 mechanical completeness to be clamped to the tube in 

 such a manner that the glass cube is situated above 

 the eye lns. 



Fig. 57. Optical construction of Abbe camera lucida. 



A very simple form is one with a small fixed mirror 

 near the tube and arranged to swing out so that the 

 eyepiece is unobstructed. 



Another form has large mirror which is adjustable 

 in its inclination to the table and distance from the eye- 

 piece. This form is provided with dark glass discs at 

 the side and below the prism to control the amount of 

 illumination from the object and the paper. The best 

 form gives full control of the mirror, has rotating discs 



169 



