[pevitteE] SCREEN IN THE PHOTO-MECHANICAL PROCESS 49 
paper may not show. Each of these exposures can be timed very accu- 
rately with trial plates—a most important consideration. 
We will see later on that the second exposure alone produces a 
vignetted screen. The mode of action in the process last described, there- 
fore, consists in impressing, before development, a vignetted screen upon 
the negative. 
Unfortunately the exposure on wbite paper obliterates, to some 
extent, the image in the shadows, and increases the difficulty of obtain- 
ing sharp dots with clean edges. In that respect the process is much 
inferior to those which we will now investigate. It has the advantage of 
requiring a very short exposure. 
III. Copying From NEGATIVES. 
Exposure through a negative gives a transparency or positive ; the 
interposition of the screen does not modify this relation. What is to be 
done with the dotted transparency does not come within the scope of this 
paper. Whether a negative is to be made from it, or whether, by some 
peculiar process, the engraved block is to be made directly from the 
transparency, I will leave for others to decide. 
To understand what is to follow, I must give briefly Messrs. Hurter 
and Driffield’s definitions of a perfect negative ; for further explanations. 
I refer to their original paper.’ 
The opacity of a film is the reciprocal of the number expressing the 
fraction of the incident light which emerges from the film. An opacity 
of ten lets one-tenth of the light pass, stopping nine-tenths of it. An 
opacity of two lets one-half of the light pass. An opacity of one does. 
not stop any light; it is perfect transparency. The density is the 
logarithm of the opacity. 
Two opacities, 0’ and O”, put together form an opacity, O, equal to 
the product of O’ and O0”. Two densities, D’ and D”, produce a density, 
D, equal to the sum of D’ and D”. 
In a perfect negative the opacities are proportional to the light 
‘intensities by which they were produced. These opacities or densities 
are reached at a certain stage of development ; -with a shorter or longer 
development, a density, D, becomes aD, a being the “development 
factor.” 
Let L be the highest light intensity of the original, and MZ the 
corresponding opacity of the negative. Another tone pL, of the original, 
is represented in the negative by the opacity MpJ, and the light trans- 
1 eau 
mitted is MpL We now place this negative in front of the camera and 


1  Photo-chemical Investigations, and a New Method of Determination of the 
Sensitiveness of Photographic Plates,” by Ferdinand Hurter, Ph.D., and N. 
Driffield—Journal of the Society of Chemical Industry, May 31, 1890. 
Sec. III., 1895. 4. 
