PHOTOGRAPHIC PLATES. 
IOI 
Cyanin (i part to 1,000 absolute alcohol) ... i part 
Liq. ammonias (i part to io water) ... ... i ,, 
Water to... .io parts 
Besides the dyes mentioned, others are available— 
ex. gr., Indophenol and malachite green, but for an 
account of these the reader must be referred to photo¬ 
graphic literature. 
With objectives specially “corrected for photo¬ 
graphy,” and with apochromatics, ordinary plates may 
be used, and in some cases they yield better results 
than plates specially sensitive to yellow. Here again, 
when there is difficulty in getting density, a thickly- 
coated slow plate should be used. The “ Sandell ” 
plates of Sandell Films and Plates, Limited, or the 
“Thickly-coated Landscape” of the same firm, are 
good examples of such plates; also the “Ordinary” 
plates of Messrs. Cadett and Neall. Where there is 
danger of over contrast, a plate of the “ Portrait ” type 
may be used ; these will be found in great numbers on 
the market. 
Exposure.. 
If we had to deal only with objects presenting so 
much obstruction to light-rays as could be accounted 
for by their opacity, it would not be difficult to arrive 
with almost unfailing precision at the proper exposure 
for the various objects to be photographed. Thus, 
with given illumination and given aperture, the ex¬ 
posure necessary is in direct proportion to the diameters 
of magnification ; and the exposure for one magnifica¬ 
tion being once determined, all other exposures could 
