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5 
1900-1901. | Orthochromatic Photography. 197 
induced Dr Vogel to study the effects of various dyes in 
increasing the sensibility of the silver salts to the more 
luminous rays of the spectrum; and so the first step in ortho- 
chromatic photography was taken. Vogel in that same year 
(1873) announced that if bromide of silver be dyed with 
certain yellow and red dyes, its normal sensitiveness to the 
yellow and green rays of the spectrum is thereby much 
increased. The work was taken up with energy and success 
by many eminent scientists, as Eder, Abney, Carey - Lea, 
Becquerel, and Waterhouse. During the time of these 
“investigations the gelatine bromide dry plates were introduced, 
-and photographers everywhere were so absorbed by this new 
‘discovery that the beginnings of orthochromatic photography 
eere almost forgotten. After a while, however, the effects of 
certain dyes were tried with much success on these new plates. 
‘In 1882-83 Tailfer & Clayton of Paris patented plates 
‘Sensitised with eosin. J. B. Edwards & Co., in 1884, intro- 
duced their isochromatic plates, and many others followed in 
quick succession. In America Carbutt, Ives, Forbes, and 
‘Weustner produced isochromatic plates. In France the 
Berothers Lumitre of Lyons have recently introduced a series of 
colour sensitive plates selectively sensitised for various parts of 
the spectrum. Special mention should be made of the ortho- 
chromatic system introduced by Messrs Cadett & Neal in 
connection with their lightning spectrum plate. 
The true action of the different dyes employed in increasing 
the sensitiveness of the silver salts is not well understood. A 
‘combination appears to take place between the silver salt and 
“the dye, but whether it is chemical or molecular only is not, I 
lieve, determined even yet. The dyes themselves belong to 
the more complicated benzene derivatives—eosin yellow, the 
potassium salt of tetra-bromo-fiuorescin, C,,H,Br,O;K,; eosin 
Sine the sodium salt of tetra-eiodo-fluorescin, C,,H,1,0,Na,; 
Bethrosin ; the potassium salt of tetra - iodo - fluorescin, 
..H, I,0;K,, and cyanin, C,,H,;N.I, which is one of the best 
Sensitisers for the orange and red rays. Malachite green, 
| together with naphthalene blue, are used as sensitisers for red, 
yellow, and green rays. 
There are two ways of using these dyes. The first is to 
add the solution of the dye to the emulsion before the plate is 
VOL. IV. iP 
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