Srcrion III., 1895. [29] Trans. R.S. C. 
1V.—The Theory of the Screen in the Photo-mechanical Process. 
By E. DEVILLE. 
ee 
(Read May 17, 1895.) 
I. THE SHADOW OF THE SCREEN. 
Although comparatively new, photo-engraving, or the “half-tone 
process,” as it is popularly called, has grown so rapidly that it has now 
become an important branch of the printing trade. An investigation of 
its theory may therefore prove of interest. 
The object of the process is to break the continuous tones of an 
original into equivalent tones consisting of white and black dots suitable 
for printing in the typographic press. For this purpose, the original is 
copied in the camera, but a short distance in front of the sensitive plate a 
screen is inserted consisting of minute Opaque and transparent figures; it is 
adjusted to project a diffused shadow over the plate, the light being strong- 
est under the transparent parts and weakest under the opaque parts, with 
varying degrees of intensity between. Whatever may be the subse- 
quent operations, the result, if they have been properly performed, is that 
all parts of the print corresponding to those parts of the photographic 
plate which have received less than a certain amount of illumination are 
covered with ink, while for all parts which have received more than the 
said amount of illumination the surface of the paper is left bare. I]lumin- 
ation means here the product of the intensity of light by the time of 
exposure. 
The first question that arises is this: How does the illumination vary 
within the shadow of the screen ? 
In copying a subject in the camera, the aperture of the lens’ diaphragm 
mgy be taken as the source of illumination ; seen from a point of the photo- 
graphic plate, its whole surface appears evenly illuminated, if the subject 
is properly focussed, the illumination being proportional to the intensity 
of the light sent by the corresponding point of the subject. 
Let ABCD, Fig. 1, be the diaphragm, and MM the plane of the 
sensitive plate. For a screen, let us take a single opaque figure, LL. 
There is, on the plate, a certain space, JJM, outside of which the whole 
of the diaphragm is visible and the illumination uniform. There is another 
space, NN, inside of which the diaphragm is invisible and where the 
illumination is zero: this is the shadow proper. In the space inside of 
MM and outside of NN, a portion only of the diaphragm is visible, and 
the illumination varies according as the diaphragm is more or less covered 
