Red prussiate of potash.......... 10 parts.
Citrate of iron and ammonia....... 1 part.
Water............................ 50 parts.

and ending with the proportions:

Red prussiate of potash............... 1 part.
Citrate of iron and ammonia.......... 10 parts.
Water................................ 50 "

I found the best plan for conducting these experiments to be: To coat a
sheet of the paper with a given mixture; to cut the sheet into strips
before exposure; to expose all the strips of the sheet, at the same
time, to the direct sunlight without an intervening negative; and to
withdraw them, one after another, at stated intervals. I found that with
each mixture there was a time of exposure which would produce the
deepest blue, that with over-exposure the blue gradually turned gray,
and that if a curve should be plotted, the abscissas of which should
represent the time of exposure, and the ordinates of which should
represent the intensity of the blue the curves drawn would have
approximately an elliptical form, so that if one knew the exact time of
exposure which would give the best result with any mixture, one might
deviate two or three minutes either way from that time without producing
a noticeable result. I have found that, with the same paper, the same
blue results with any good proportions of the chemicals named, provided
a sufficient weight of both chemicals is applied to the surface; that an
excess of the red prussiate of potash renders the preparation less
sensitive to light, and very much lengthens the necessary time of
exposure; that the prints are finer with some excess of the red
prussiate; that an excess of the citrate of iron and ammonia hastens the