54 ROYAL SOCIETY OF CANADA 
ascertained by trial. After development the screen is found to consist of 
semi-transparent and semi-opaque dots alternately disposed ; examining it 
under a moderately strong microscope, the central part of the semi- 
transparent dots must be thin, but not clear glass. The presence of 
minute spots of clear glass indicates under-exposure. Under a strong 
microscope it will be found that, even with correct exposure, there are 
very minute dots of clear glass in the centre of the semi-transparent dots ; 
their size must be kept down by a full exposure. A screen not sufficiently 
developed does not reproduce the deep shadows ; they come out solid 
black in the print, and the shadows are generally too dark. A screen too 
much developed gives a print in which the shadows are too light ; there 
is no solid black. Under-exposure is indicated by the absence of the 
finest white dots in the print. A long exposure, within reasonable limits, 
gives a dense screen, but correct gradations; the only disadvantage is 
that it takes a long time to print. The collodion or emulsion must be of 
a kind giving soft negatives with good gradation. Special care must, of 
course, be taken to have the plate perfectly clean and everything in first 
class working order, so as to secure a screen free from defects. 
In theory, the vignetted screen is the most perfect one for the photo- 
mechanical process, because correct prints may be obtained from thin or 
intense transparencies by using thin or intense screens, while in copying 
from a negative through a chess-board screen, the negative must be of 
right density to give a correct print. 
V. Tur DIAPHRAGMS. 
The proper shapes for the apertures in the diaphragms, or “ stops,” 
as they are more generally called, have been described in the course of 
this paper; we have now to consider their adjustment. 
The square is the only shape adapted to the cross-lined screen with 
its adjustment has been given in equation (4) : 
ae 
NA 
But this is not the proper way to use a cross-lined screen. Its opaque 
and transparent lines should be of equal width, and it should be employed 
with double aperture stops, so as to work like a chess-boardscreen. The 
adjustment of the diaphragm is then given by the formula: 
unequal lines ; 
et, 
Ji 
toe 9n À’ 
or 
Ie 2 
DE fe sss 16 
= ing (16) 
The distance between the centres of the pair is : 
Cll / Su 
