Ch. VI] DRAWING BY THE AID OF PHOTOGRAPHY 181 



vantage that the lines are perfectly distinct in every stage of the 

 work. If drawn on the face of the print the blue obscures the pencil 

 lines more or less while the drawing is being made. 



For many objects it makes no great difference whether the picture 

 is reversed or not, but in some cases there should be no reversal. 

 One can easily make the picture so that the final picture will be 

 erect as follows: Print the negative so that the photograph will be 

 reversed; then when the line drawing is made on the back of the 

 print the line drawing will be erect. Of course if the lines are made 

 on the face of the print as with the blue print (§ 288) the print must 

 be erect. To get erect prints turn the film side of the negative toward 

 the sensitive paper as with contact printing. For reversed prints 

 turn the glass side of the negative toward the paper. 



This method of drawing is applicable for all sorts of objects, the 

 photographic print serving to give all the outlines and proportions. 

 No measurements need be made. Then by drawing the outlines 

 on the back of the print, one can do all shading as if no picture were 

 on the opposite side. It is of course not necessary for highly trained 

 artists, but is of the greatest assistance for amateurs; and most 

 biologists are amateurs. 



In finishing the drawing, the object should be in view to make 

 certain that the drawing is accurate. 



§ 289a. Diaphragming the objective and the use of a concentrated filament 

 lamp. — In the above directions a first-class photographic objective was a.-Mimed. 

 If now one has not a first class objective or for any reason it is desirable to dose 

 the diaphragm more or less, the unobstructed cone of light cannot be used, but 

 there must be a diffuser like ground-glass or milky fdass put between the source 

 of light and the negative to be projected. With such a diffuser one can close the 

 diaphragm as desired. Of course the addition of the diffuser and the closing of 

 the diaphragm will necessitate a longer exposure. 



If instead of an arc lamp a concentrated filament stereopticOD lamp is used one 

 must also employ a diffuser or the shadows between the filaments of the lamp will 

 give rise to inequalities in the print. The diffuser can be pul between the con- 

 denser lenses or between the lamp and the condenser. It must be far enough from 

 the negative so that the grain of the ground-glass will not show in the print (.§ 362). 



§ 290. Retouching photographs for halftone reproduction. — For 

 pictures of animals, organs, and dissections to be reproduced by the 

 halftone process, very successful drawings can be made as follows: 

 Arrange the object as it is to appear in the finished drawing; light it 



