126 DRAWING WITH THE MICROSCOPE \_CH. V 



59 and the prism loosened and raised or lowered to the proper level ; 

 but in doing this one should avoid setting the prism obliquely to the 

 mirror. 



In the latest and best forms of this camera lucida special arrange- 

 ments have been made for raising or lowering the prism so that it may 

 be used with equal satisfaction on oculars with the eye-point at differ- 

 ent levels, and the prism is hinged to turn aside without disturbing 

 the mirror. 



One can determine when the 

 camera is in a proper position by 

 looking 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 

 FIG. 113. One of the latest and pos ition. If one side of the field is 

 best forms of the Abbe Camera Lucida d fc ^ h rf . Qne ^ 



(Bausch & Lomb Optical Co.). 



of the center ; if the field is consid- 

 erably 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 below the eye- 

 point. 



183. Arrangement of the Mirror and the Drawing Surface. 

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

 scope (Fig. 109). On a vertical microscope if the mirror is set at an 

 angle of 45, the axial ray will be at right angles with the table top or a 

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

 conditions will be 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. 1 14). But as the axial ray from the mirror 

 to the prism must still be reflected horizontally, it follows that the axial 

 ray will no longer form an angle of 90 degrees with the drawing sur- 

 face, but a greater angle. If the mirror is depressed to 35, then the 

 axial ray must take an angle of 110 with a horizontal drawing surface 

 (see the geometrical figure Fig. 114, A). To make the angle 90 again, 

 so that there shall be no distortion, the drawing board must be raised 

 toward the microscope 20. The general rule is to raise the draw- 

 ing board twice as many degrees toward the microscope as the 

 mirror is depressed below 45. Practically the field for drawing 



