520 HISTOLOGY 



purple a combination of the darkest and lighter than pure red since 

 it has the brighter yellow mixed with it. Thus the various colors may be 

 suggested in black and white, and the contrast between blue nuclei and 

 red protoplasm can be carefully preserved in the drawing. This is facili- 

 tated by the use of pencils of varying degrees of hardness "H" and 

 "3 H" for dark structures, and "6 H" for pale areas. Soft pencils, which 

 rub, should not be used. 



Before beginning a drawing, the specimen should be carefully looked 

 over, to find the place most worthy of such attention. The time which 

 the drawing is to take must be considered, and a small area may be 

 found which combines features elsewhere scattered about the specimen. 

 The entire field is rarely, if ever, to be drawn; and the figures should not 

 be encumbered with surrounding circles. 



The magnification of the drawing is next to be decided upon. The 

 form of a gastric gland and the structure of its cells, for example, cannot 

 profitably be included in a single drawing. General features, such as the 

 forms of glands, should be represented in "low power" sketches. "Low 

 power" as here used does not necessarily refer to the lenses employed, 

 but means that the drawing is on such a scale that the nuclei appear merely 

 as spots, round or elongated as the case may be. Often, however, such 

 a drawing shows features which can be clearly observed only with high 

 power lenses. "High-power drawings" are those which present details 

 of nuclear and protoplasmic structure. 



Usually in studying an organ, it is desirable to make a general low- 

 power sketch showing the arrangement of its lobules or layers, and to 

 supplement this by high-power drawings of the most significant cells or 

 tissues. In these, which are the final test of a student's keenness of ob- 

 servation, no details of cellular structure are too minute for careful repre- 

 sentation, and "the difficulty of observing them proves not the merit of 

 overlooking them." 



Having selected a field and decided upon the magnification, the out- 

 lines of the parts should be sketched lightly, with a soft pencil, and cor- 

 rected until accurate. As finally made, they should be definite clean lines, 

 not pieced out, representing the boundaries of layers, nuclear membranes, 

 cell 'walls when present, cuticular surfaces, and the like. Having com- 

 pleted the outline, shading should be undertaken, to differentiate substance 

 from empty space, and to indicate the nature of the substance. In high- 

 power drawings protoplasmic texture must be faithfully reproduced ho- 

 mogeneous, finely granular or coarsely granular; if the granules are not dis- 

 tinct enough to be counted, they should not be readily countable in the 

 drawing. If definite walls are absent from the specimen, they should 

 not be drawn, but the shaded areas of the finished drawing should end 

 abruptly without a bounding line. 



