My color-screen consists of a small plate-glass tank, having a space of 3/16 of of an inch between the glass, filled with a solution of bichromate of potash about one grain strong.  I place the tank in front of the lens, in contact with the lens-mount.  The advantage of this tank and solution is that it can be more easily obtained than yellow plate glass, and the color can be adjusted to meet any requirement.

The plates require about three times as much exposure through the yellow screen as without it, and may be developed with the ordinary alkaline pyro-developer.

[Illustration:  IVES’ PROCESS OF ISOCHROMATIC PHOTOGRAPHY.]

In order to illustrate the value of this process, I made two photographs of a highly-colored chromo-lithograph, representing a lady with a bright scarlet hat and purple feather, a yellow-brown cape and a dark-blue dress.  One, by the ordinary process, represents the blue as lighter than the yellow-brown, the bright scarlet hat as black, and the purple feather as nearly white.  The other, by the chlorophyl process, reproduces all colors in nearly the true proportion of their brightness, but with a slight exaggeration of contrast produced purposely by using a too-strong color solution in the small tank.

I also made two landscape photographs, one by the ordinary process, and the other by the chlorophyl process, exposing them simultaneously.  In the ordinary photograph, distant hills are lost through overexposure, yet the foreground seems underexposed, and yellow straw-stacks and bright autumn leaves appear black.  In the chlorophyl photograph, the distant hills are not overexposed, 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.