60 ROYAL SOCIETY OF CANADA 
There may be cases where it is desirable to modify the gradations of 
the print ; this can be done by remembering the following rule : 


Fie. 30. 
The dots, black and white, are enlarged by making the points of the 
star more acute in the diaphragm’s aperture; they are reduced by 
making the points more obtuse. The squares of the middle tone and the 
finest dots are not changed. 
VI. ConcLusIons. 
From these investigations we conclude that there are three methods 
for translating correctly the continuous half-tones of an. original into 
tones consisting of white and black dots. 
Ist. By copying from the original, giving a first exposure without a 
screen on the original, and a second exposure, through a chess-board or 
cross-lined screen, on a sheet’of white paper or other source of uniform 
illumination. This method does not produce dots with edges as sharp 
and clean as the other two methods. 
2nd. By copying from a negative through a chess-board or cross- 
lined screen. A negative of proper density produces a perfect result, but 
it [gives a transparency and therefore requires some new process for 
etching the block. 
3rd. By copying from the original or printing through a trans- 
parency with a vignetted screen in contact with the photographic plate. 
