He afterward admitted that only such dyes as are capable of
entering into chemical combination with the silver-bromide proved capable
of increasing its sensitiveness to color, but he held to the theory that
the optical properties of the compound were the cause of its
color-sensitiveness.
I have shown that the color-sensitiveness can be produced by treatment
with an organic compound which has none of the optical properties
characteristic of dyes; and that chlorophyl, which absorbs only red
light, greatly increases the sensitiveness also to yellow and green.
There is, therefore, good reason to doubt if the color-sensitiveness is
ever due to the optical properties of the dye or combination.
Attempts have been made to produce isochromatic gelatine dry plates
which, while many times more sensitive to white light than my chlorophyl
plates, shall also show the same relative color-sensitiveness. Such
plates would be very valuable but for one fact: it would be necessary to
prepare and develop them in almost total darkness. Gelatine bromide dry
plates extremely sensitive to yellow, but _comparatively insensitive to
red_, might be used to advantage in portrait and instantaneous
photography, because they could be safely prepared and developed in red
light; but when truly isochromatic photographs are required, the time of
exposure must be regulated to suit the degree of sensitiveness to red,
which cannot safely be made greater than I have realized with my
chlorophyl process.
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