40 ROYAL SOCIETY OF CANADA 
even then, not without precise adjustment of diaphragm and screen. The 
general form of grain is reticulated ; the small dots may or may not be 
isolated, but the larger ones are connected, the black ones by thin black 
lines, or the white ones by thin white lines. These thin lines have an ill 
effect on the printing qualities of the block; the black ones spread and 
the white ones clog in printing. Being so thin, they are liable to break 
in development and to cause uneven tints. These lines do not appear or 
disappear gradually; while the dots are still a considerable distance 
apart, a very slight change of illumination causes the connecting lines to 
shoot out between them. This sudden accession to the area of the dots 
destroys the continuity of the gradations. 
The adjustment given in Fig. 15 produces a print of which the middle 































































































































































































Jthigeae lc. 
tone consists of equal black and white squares. To prove this, move the 
perspective of the diaphragm to the position ABCD, Fig. 16; the visible 
portion is : 
S = (a + by, 
a being one-half of PO and b the distance EN. Moving now the centre 
E of the perspective around the squares FLMG and HFG E, it is found 
that the area of the visible portion remains constant. The sides of the 
squares being lines of equal illumination, these squares are the dots of the 
