CH. /'] DRAWING WITH THE MICROSCOPE 147 



ing board should be elevated 20 toward the microscope. Further- 

 more it is necessar)^ in using an elevated drawing board to have the 

 mirror bar project directly laterally so that the edges of the mirror 

 are 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 avoids right to left distortion. If one has a 

 micrometer ruled in squares (net micrometer} the distortion pro- 

 duced 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 

 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 10 and 20 determined 

 with a protractor. The drawing 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. 



205. 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 microscope. All that is requisite is to be sure that the fun- 

 damental 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 accomplished as follows: The drawing board is raised 

 toward the microscope twice as many degrees as the mirror is de- 

 pressed below 45 ( 204), then it is raised exactly as many degrees 

 as the microscope is inclined, and in the same direction, that is, so 

 the end of the drawing board shall be in a plane parallel with the 



