5Q 



MAGNIFICATION AND DRAWING. 



lows that the axial ray will no longer form an angle of go degrees with 

 the drawing surface, but a greater angle. If the mirror is depressed 

 to 35 , then the axial ray must make an angle of no° with a hori- 

 zontal drawing surface, see the geometrical figure, (Fig. 33). ■ 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 micro- 

 scope, at 45 , at 40 or at 35 . In the first case (45 mirror) the draw- 

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

 drawing surface should be elevated io°, and in the third case (35 mir- 

 ror) 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, other- 

 wise 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 horizontal 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 Iposition of the wedge for io° and 20 the protractor need be used 

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

 proper elevation. 



§ 125. Abbe Camera and Inclined Microscope. — It is very fatigu- 

 ing 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 



