verexposed, nor is the foreground underexposed; the yellow
straw-stacks appear nearly white, and bright autumn leaves contrast
strongly with the dark green about them.

To test the relative color-sensitiveness of plain emulsion plates, plates
stained with eosine, and plates stained with the blue-myrtle chlorophyl,
I exposed one of each kind through the same yellow screen, giving each
five minutes exposure, on the same piece of copy, which was the
chromo-lithograph already described. The plain emulsion plate showed only
the high lights of the picture, after prolonged development. The eosine
plate was underexposed, but brought up everything fairly well except the
scarlet hat, which came up like black. The chlorophyl plate was
overexposed, brought out all colors better than the eosine plate, and
gave full value to the bright scarlet of the hat, the detail in which was
beautifully rendered.

Dr. Vogel advanced the theory that silver-bromide is insensitive to
yellow and red, because it reflects or transmits those colors; and that
it becomes sensitive when stained, because of the optical properties of
the dyes. 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
exp