146 DRAWING WITH THE MICROSCOPE [ CH. r 



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 

 arrangements 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 different levels, and the prism is hinged to turn aside with- 

 out disturbing the mirror (Figs. 128, 132). 



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

 looking into the microscope through it. If the field of the micro- 

 scope 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. <?., it is 

 above or below the eye-point. 



204. Arrangement of the Mirror and the Drawing Sur- 

 face. The Abbe camera lucida was designed for use with a vertical 

 microscope (Fig. 124). 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 a 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. 129). But as the 

 axial ray from the mirror to the prism must still be reflected hori- 

 zontally, it follows that the. axial ray no longer forms an angle of 

 90 degrees with the drawing surface, but a greater angle. If the 

 mirror is depressed to 35, then the axial ray takes an angle of 110 

 with a horizontal drawing surface (see the geometrical figure Fig. 

 129 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 drawing board twice 

 as many degrees toward the microscope as the mirror is 

 depressed below 45. Practically the field for drawing can 

 always be made free of the stage of the microscope, at 45, at 40, 

 or at 35. In the first case (45 mirror) the drawing surface should 

 be horizontal, in the second case (40 mirror) the drawing surface 

 should be elevated 10, and in the third case (35 mirror) the draw- 



