XXVI 



REMARKS TO THE STUDENT 



are permitted. A further device to show the parts in contrast is the 

 use of black and white, as in Fig. 2 F. The simplest way to do this 

 is by using the pencil to darken limited parts of the figure. If too 

 many parts are thus darkened the contrasts will be lost. Or one may 

 use the common red and blue crayons, but this should be done with 

 great care and the color kept within the boundaries of the blood ves- 



FiG. 4. — Drawing of a nematode, in which the perspective is shown by lines 

 at the posterior end of the body and by shading in the head region. 



(From N. A. Cobb, 1914, Yearbook, V. S. Dept. Agriculture.) 



sel or other parts. Experience shows that few students will do crayon 

 work with sufficient care not to merely smudge; hence it is not favored 

 by most teachers. In ink drawings where this technique of darkening 

 is used, the special parts are merely painted in, with care not to over- 

 lap the outline and to fill the area completely, as in Fig. 2 F. 



An even more effective means of indicating the third dimension 

 is by what is called shading. This is not mere darkening, as cxi^lained 

 in the previous paragraph, but rather the distribution of pencil or ink 



