i66 



DRAWING WITH A CAMERA LUCIDA 



[Ch. VI 



the mirror to the prism must still be reflected horizontally, it follows 

 that the axial ray no longer forms an angle of 90° with the drawing 



surface, but a greater 

 angle. If the mirror is 

 depressed to 35°, then 

 the axial ray makes 

 an angle of 110° with 

 a horizontal drawing 

 surface (fig. loi B): 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 de- 

 pressed below 4s". 

 Practically the field 

 for drawing can al- 

 ways be made free of 

 the stage of the micro- 

 scope, at 45°, at 40°, 

 or at 35°. In the first 

 case (45° mirror) the 

 drawing surface should 

 be horizontal, in the 

 second case (40° mir- 

 ror) the drawing sur- 

 face should be elevated 

 10°, and in the third case (35° mirror) the drawing board should be 

 elevated 20° toward the microscope. Furthermore it is necessary 

 in using an elevated drawing board to have the mirror bar of the 



Fig. ioi. Diagram of the Abbe Camera Lu- 

 cida with the drawing sxirface elevated to 

 Make the Axis Perpendictoae with Depressed 

 Mirror. 



A, Axis, Axis The axial ray from the microscope 

 and from the drawing surface. 



Ocular The upper part of the microscopic ocular. 



Mirror The minor of the camera lucida; it is de- 

 pressed from 45° to 35° to make the axis from the 

 drawing surface perpendicular to the axis of the micro- 

 scope. 



A — B The drawing surface elevated 20°; that is, 

 twice as many as the mirror is depressed below 45°. 



W Wedge under the drawing board. 



P, P The drawing pencil and the prism of the 

 camera lucida. 



Q Quadrant of the mirror. 



B Geometrical figure to show why the drawing 

 board must be raised twice as many degrees as the 

 mirror is depressed to keep the axial ray perpendicu- 

 lar to the drawing surface. 



