CH V,~\ DRAWING WITH THE MICROSCOPE. 113 



The Abbe camera lucida was designed for use with a vertical microscope 

 (Fig. 102). 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 draw- 

 ing board which is horizontal, and a drawing made under these condi- 

 tions would 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. 106-107). 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 1 io° with a horizontal drawing surface ; 

 see the geometrical figure, (Fig. 107). 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 

 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 io°, 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 

 project directly laterally so that the edges of the mirror will be in 

 planes parallel with the edges of the drawing board, otherwise there 

 will be front to back distortion, although the elevation of the drawing 

 board would avoid right to left distortion. If one has a micrometer 

 ruled in squares (?iet micrometer) the distortion produced by not hav- 

 ing the axial ray at right angles with the drawing surface may be very 

 strikingly shown. For example, set the mirror at 35 and use a hori- 

 zontal drawing board. With a pencil make dots at the corners of some 

 of the squares, and then with a straight edge connect the dots. The 

 figures will be considerably longer from right to left than from front to 

 back. Circles in the object would appear as ellipses in the drawings, 

 the major axis being from right to left. 



The angle of the mirror may be determined with a protractor, but 

 that is troublesome. It is much more satisfactory to have a quadrant 

 attached to the mirror and an indicator on the projecting arm of the 

 mirror. If the quadrant is graduated throughout its entire extent, or 

 preferably at three points, 45 °, 40 and 35 °, one can set the mirror at a 

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