Ch. VI] DRAWING WITH A CAMKRA LUCTDA 165 



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

 lucida. The explanation of the apparent location of the image, etc., on the draw- 

 ing board with the Abbe camera lucida is that the attention is concentrated upon 

 the drawing surface rather than upon the object under the microscope. With 

 some observers it is possible to make the image appeal under the microscope or 

 on the drawing surface at will by concentrating the attention on one position or 

 the other. (Dr. \V. B. Pfflsbury). 



§ 272. Arrangement of the camera lucida prism. — In placing 

 this camera lucida over the ocular for drawing or the determination 

 of magnification, the center of the hole in the silvered surface is 

 placed in the optic axis of the microscope. This is done by properly- 

 arranging the centering screws that clamp the camera to the micro- 

 scope tube or ocular. The prism must not only be centered to the 

 axis of the microscope, but it must be at the right level or more or 

 less of the field will be cut off. In all the good modern forms of 

 this camera lucida it is fastened to the tube of the microscope by a 

 clamp which enables one to raise or lower it so that it may be at 

 the right position with reference to the eye-point of the ocular being 

 used (§57). 



One can determine when the camera is in a proper position by look- 

 ing into the microscope through it. If the field of the microscope 

 appears as a circle and of about the same size as without the camera 

 lucida, then the prism is in a proper position. If one side of the 

 field is dark, then the prism is to one side of the center; if the field 

 is considerably smaller than when the prism is turned off the ocular, 

 it indicates that it is not at the correct level, i.e., it is above or too 

 far below the eye-point. 



§ 273. Arrangement of the mirror and the drawing surface. — 

 The Abbe camera lucida was designed for use with a vertical micro- 

 scope (fig. 100). On a vertical microscope if the mirror is set at an 

 angle of 45 , the axial ray is at right angles with the table top or 

 drawing board which is horizontal, and a drawing made under these 

 conditions is in true proportion and not distorted. The stage of 

 most microscopes, however, extends out so far at the sides that with 

 a 45 mirror the image appears in part on the stage of the microscope. 

 In order to avoid this the mirror may be depressed to some point 

 below 45 , say at 40 or 35 (fig. 101). But as the axial ray from 



