46 ROYAL SOCIETY OF CANADA 
The shorter diagonal of the rhombus-shape extensions is 0:19 of the 
diagonal of the central square ; the longer diagonal of the rhombus is 
twice the length of the shorter one. Each of the smaller squares is equal 
to one-quarter of the area of the central square. The dots produced by 
this diaphragm are approximately those of Fig. 22. 
A method much in vogue among operators consists in giving the 
exposure through several square diaphragms of various sizes. With a 
chess-board screen, the procedure may be as follows : 
The first or preliminary exposure is given through a diaphragm 
AB, Fig. 23, placed at a distance, OP, from the optical axis of the lens 
equal to the diagonal of the aperture. The length of this diagonal is 
deduced from equation (16). The pur- 
pose of this exposure is to give the 
illumination which is lacking under the 
centre of the opaque squares of the 
‘screen. The illumination due to this 
diaphragm is maximum under the cen- 
tre of these squares, while it is null 
under the centre of the transparent 

Fig. 22. Fia. 23. 
squares. The preliminary exposure should be just sufficient to give ¢ 
trace of the highest lights, if the plate were fully developed and intensi- 
fied. 
The main exposure is given through the diaphragm CD, equal to AB, 
and placed in the centre of the lens; it is equal to two and a half times 
the preliminary exposure. This exposure determines the size of the dots 
in the lights, but the shadows are still veryimperfect. If the plate were 
now developed and a block made, the middle tone of the print would be 
found to represent the tone, 0°57 L, of the original ; all tones less bright 
than 0-4 Z would come out as solid black. 
