44 PRACTICAL HISTOLOGY. 



ment of the pencil. This continuous smooth shading, so 

 necessary for giving a fine effect, may be readily produced 

 by rubbing the pencil drawing with an artist's " stump," 

 consisting of a firm cone of soft paper on the end of a 

 stick. The student should draw the object just as he sees 

 it, without altering its apparent size. In making drawings 

 of complicated objects, for the purpose of publication, it 

 is often necessary to reduce the size, and to fill in the 

 details of one drawing from many specimens, but with this 

 the beginner has nothing to do. The only practice advisable 

 in his case is to draw things just as he sees them. It is 

 important to practise even the drawing of outlines without 

 the aid of a camera. The details of objects must indeed 

 always be filled in without such aid, nevertheless it is often 

 difficult to get the exact size and the general form without 

 a camera. To all drawings a statement of the magnifying 

 power should be added, e.g. milk corpuscles x 300 ; and 

 in the case of things very difficult to see it is well to state 

 the objective and ocular employed, not only their numbers, 

 but also the names of their makers. 



a. Chevalier's camera lucida is an exceedingly good one 

 (Fig. 38). The eye-piece of the microscope is removed, 

 and the camera inserted in its place, and fixed in any 

 position with a screw (s). The rays penetrate a prism (/), 

 undergo total internal reflection, emerge, are collected by a 

 special eye-piece, and penetrate a second prism (/), where 

 they are reflected to the eye of the observer, to whom the 

 object appears to lie upon the table. The outlines of the 

 object may be traced with a pencil, as shown in the figure. 

 It must be remembered, however, that the pencil cannot 

 possibly be seen through the prism. The rays from the 

 pencil have to pass its margin, and so reach the pupil. 

 One-half of the pupil has therefore to be devoted to the 

 rays from the prism, and the other to rays from the pencil 

 and the paper. With Chevalier's camera the stage of the 

 microscope is kept horizontal, so that it is perfectly service- 

 able in the case of fluids. The paper and the point of the 

 pencil should be eight or ten inches from the eye. With 

 a microscope having a tube ten inches long, the paper 

 should be placed on a box on a level with the stage. 



