196 Orthochromatic Photography. [Sess. 
are found in the greater portion of the spectrum, though most 
apparent in the yellow space and close by—may it not also be 
that, in the case of the chemical rays, they, too, are found © 
everywhere, even in the yellow and red, though much more 
energetic in the blue and violet? In order to test this an 
ordinary plate might be exposed to the spectrum, but the pre- 
caution taken that the blue and allied rays should be cut out 
by means of a piece of yellow glass, or a solution of chromate 
of potassium. Here is a plate (Plate XXIII. 2, Imperial — 
special-rapid) that has been so exposed, and you see with what 
result. There is now a deposit even as far as the réd, though 
in degree it is faint. To obtain this an exceedingly long 
exposure was necessary—one hour and forty minutes—and if 
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oF re 
TO a a En se 
SS Bee a a 
the blue rays had not been cut out the plate would have been @ 
spoiled by halation and fogging caused by reflected blue light 
in that long time. This experiment shows that some approach 
to correct representation of colour luminosity may be obtained 
even with an ordinary plate by the artifice of interposing a 
properly coloured light filter, though for practical purposes it — 
is quite useless, owing to the imordinate length of exposure 
required. If this faint sensitiveness of silver salts to the red, 
yellow, and green rays could be much increased, then ortho- 
chromatic photography would be a possibility; for, by using a : 
plate so sensitised, and cutting off the two active blues by 
suitable light-filters, the deposit on the plate could be made 4 
more nearly to follow the luminosity values of the different 
parts of the spectrum. 
In 1873, by a happy chance, some (collodion) bromide 
dry plates, prepared by Colonel Stuart-Wortley of England, and — 
stained with some yellow substance, came into the hands of © 
a man of observation. This was Dr H. W. Vogel of Berlin. 
He noticed that these plates gave a more correct rendering of _ 
green and yellow colours than any he had seen, He did not 
find this sensitiveness to yellow and green in the collodion © 
bromide plates prepared by himself, and so judged there must be | 
something in Colonel Stuart-Wortley’s plates that was causing 
the difference. To test this he washed one of the plates in — 
alcohol and so removed the yellow stain, and he found that — 
after this treatment the plate was no longer as sensitive to — 
yellow and green as before. The result of this experiment — 
