128 D RAWING WITH THE MICROSCOPE [CH. V 



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 (net micrometer) the distortion produced by not having 

 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 

 known angle in a moment, then the drawing board can be hinged and 

 the elevation of io° and 20 determined with a protractor. The draw- 

 ing board is very conveniently held up by a broad wedge. By marking 

 the position of the wedge for 10° and 20 the protractor need be used 

 but once, then the wedge may be put into position at any time for the 

 proper elevation. 



§ 184. Abbe Camera and Inclined Microscope. — It is very 

 fatiguing to draw continuously with a vertical microscope, and many 

 mounted objects admit of an inclination of the microscope, when one 

 can sit and work in a more comfortable position. The Abbe camera is 

 as perfectly adapted to use with an inclined as with a vertical micro- 

 scope. All that is requisite is to be sure that the fundamental law is 

 observed regarding the axial ray of the image and the drawing surface, 

 viz., that they should be at right angles. This is very easily accom- 

 plished as follows : The drawing board is raised toward the microscope 

 twice as many degrees as the mirror is depressed below 45° (§ 183), 

 then it is raised exactly as man}' degrees as the microscope is inclined, 



