5i 



MAGNIFICA TION AND DRA WING. 



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

 viz., that they should be at right angles (PI. IV). This is very easily 

 accomplished as follows : The drawing board is raised toward the mi- 

 croscope twice as many degrees as the mirror is depressed below 45 

 (§ 124) 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 stage of the microscope. 

 The mirror must have its edges in planes parallel with the edges of the 

 drawing board also (Fig. 34).* 



§ 126. Drawing with the Abbe Camera Lucida. — (A) The light 

 from the microscope and from the drawing surface should be of nearly 

 equal intensity so that the image and the drawing pencil can be seen 

 with about equal distinctness. This may be accomplished with very low 

 powers (18 mm. and lower objectives) by covering the mirror with 

 white paper when transparent objects are to be drawn. When opaque 

 objects, like dark insects, are to be drawn it may be necessary to con- 

 centrate light upon them with a condensing lens or concave mirror. 

 For high powers it is best to use an Abbe illuminator. Often the light 

 may be balanced by using a larger or smaller opening in the dia- 

 phragm. One can tell which field is excessively illuminated, for it is 

 the one in which objects are most distinctly seen. If it is the micro- 

 scopic, then the image of the microscopic object is very distinct and 

 the pencil is invisible or very indistinct. If the drawing surface is too 

 brilliantly lighted the pencil can be seen clearly, but the microscopic 

 image will be very obscure. 



If the drawing surface is too brilliantly illuminated, it may be shaded 

 by placing a book or a ground glass screen between it and the window, 

 also by putting one or more smoked glasses in the path of the rays from 

 the mirror (Fig. 30, G). If the light in the microscope is too intense, 

 it may be lessened by using the white paper over the mirror, or by a 

 ground glass screen between the microscope mirror and the source of 

 light (Piersol, Amer. M. M. Jour., 1888, p. 103). It is also an excel- 

 lent plan to blacken the end of the drawing pencil with carbon ink. 

 Sometimes it is easier to draw on a black surface, using a white pencil 

 or style. The carbon paper used in manifolding letters, etc., maybe 

 used, or ordinary black paper may be lightly rubbed on one side with a 

 moderately soft lead pencil. Place the black paper over white paper 

 and trace the outlines with a pointed style of ivory or bone. A corre- 

 sponding dark line will appear on the white paper beneath. (J. R. M. 

 S., 1883, p. 423). 



(A) It is desirable to have the drawing paper fastened with thumb 



*This method of using the Abbe camera lucida on an inclined microscope was 

 devised by Mrs. Gage. 



