41 8 HISTOLOGY. 



In working with the Abbe condenser the flat surface of the mirror 

 should be uppermost. 



The objectives must never be scratched. Lens paper or fine linen 

 should be used to wipe them. If they are soiled with damar they should 

 be wiped with a cloth moistened with xylol. Since the lenses are mounted 

 in balsam, xylol must be applied to them cautiously. 



In lifting the microscope it should never be taken by any part above 

 the stage; the pillar should be grasped below the stage. 



DRAWINGS. 



Drawings should be made of all the significant structures observed; 

 the structure should be observed however, before any drawing is attempted. 

 In other words a thorough study of the specimen should precede the draw- 

 ing. The nondescript character of many drawings seems due to the fact 

 that the student had nothing definite in mind to portray. It is true never- 

 theless that the repeated observation made while a careful drawing is in 

 progress, reveals many details which would otherwise be overlooked. 



The drawings should be simple but exact, made and shaded with a 

 hard (6 H) lead pencil having a sharp point. They should not be encum- 

 bered with surrounding circles. The parts are to be labelled in one's 

 plainest handwriting (not printing); and the terms should be explicit. 

 A line proceeding from a mass of chromatin within a cell nucleus ought not 

 to be labelled either cell or nucleus but chromatin. Some knowledge of 

 drawing is very desirable although perspective is scarcely involved in histo- 

 logical work. The lightly colored structures should be made lighter and 

 the dark ones darker than they appear, to preserve the contrasts of the 

 stains. The lines should be few and made with assurance, not pieced 

 out as if one were feeling his way. Every line should correspond with some 

 structure; if a cell has no wall, the even or granular shading representing 

 its protoplasm should end abruptly, but without a bounding line. 



RECONSTRUCTIONS. 



There is an important arrangement of mirrors (Abbe's camera hicida } 

 for drawing the outlines of sections. It is attached to the microscope so 

 that the image of the section beneath the objective appears spread upon the 

 drawing paper. The paper is on the table beside the base of the microscope. 

 On looking through the camera into the microscope one can see the pencil 

 point, as it is made to trace the outline on the paper. In this way a suc- 

 cession of serial sections may be drawn with uniform magnification. The 

 magnification is determined by substituting a stage micrometer for the slide 

 of sections. The micrometer is a slide upon which i mm., with subdivisions 



