CH. VI] DRAWING WITH A CAMERA LUCIDA 167 



camera lucida project directly laterally so that the edges of the mirror 

 are 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 avoids right to left distortion. If one has a microm- 

 eter ruled in squares (net micrometer) (fig. 65, 97), 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 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. 



274. Abbe camera and inclined microscope. It is very fati- 

 guing 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 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 

 accomplished as follows: The drawing board is raised toward the 

 microscope twice as many degrees as the mirror is depressed below 

 45 ( 2 73) 5 then it is raised exactly as many degrees as the micro- 

 scope is inclined, and in the same direction, that is, so that the end 

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



