[Footnote: Read at the stated meeting of the Franklin Institute, March
18, 1885.]
By FRED. E. IVES.
It is well known that the ordinary photographic processes do not
reproduce colors in the true proportion of their brightness. Violet and
blue photograph too light; green, yellow, orange and red, too dark. For a
long time it was believed to be impossible to remedy this defect; and
even when it became known that bromide of silver could be made more
sensitive to yellow and red by staining it with certain dyes, the subject
received very little attention, because it was also known that the
increase of sensitiveness was too slight to be of practical value in
commercial photography.
Dr. H.W. Vogel, who was one of the first, though not the first, to devote
attention to this subject, announced, in 1873, that he had succeeded in
making a yellow object photograph lighter than a blue or violet one, by
using a silver-bromide plate stained with coraline, and exposed through a
yellow glass. The plate showed no increased sensitiveness to red, and the
experiment, although of considerable scientific interest, did not
indicate a practically useful process.
In the spring of 1878 I became interested in this subject, and tried to
discover a method of producing plates which should be sensitive to all
colors, and capable of reproducing them in the true proportion of their
brightness. I commenced by trying nearly all the color sensitizers which
had already been suggested, in order to learn which was the best, and
then, if possible, _why_ it was the best, as a guide to further research.
Chlorophyl was the only thing I tried which was sufficiently sensitive to
red to offer any encouragement in that direction; but the solution which
I obtained was weak and unstable, and far from being a satisfactory color
sensitizer. Hoping to obtain a better solution with which to continue my
experiments, I made extracts from many kinds of leaves, and found that a
solution from blue myrtle leaves looked better and kept better than any
other, and when it was applied to the silver-bromide plates they became
remarkably sensitive, not only to all shades of red, but also to orange,
yellow, and green. By placing in front of the lens a color-screen
consisting of a small glass tank containing a weak solution of bichromate
of potash, to cut off part of the blue and violet light, I obtained, with
these chlorophyl plates, the first photographs in which all colors were
reproduced in the true proportions of their brightness. But my chief
desire at that time was to realize a method of producing from any object
in colors a set of three negatives, in one of which the shadows should
represent the blue of the original, in another the yellow, and in another
the red, in such a manner that transparent pigment prints from these
negatives--blue, yellow, and red--would, when superimposed on a white
surface, represent not only the lights and shadows, but also the colors
of the object. This had already been attempted by others, who failed
because their plates were not sufficiently sensitive to red and yellow.
Having succeeded perfectly in my undertakings, I published my discovery
in 1879,[1] explaining how to prepare and use the chlorophyl plates, in
connection with the yellow screen, for the purpose of securing correct
photographs of colored objects.[2]
[Footnote 1: _Philadelphia Photographer_, December, 1879, p. 365.]
[Footnote 2: I intended this publication to be a very full and explicit
one, and it was sufficiently so to be perfectly understood by most who
saw it; but some may think I did not sufficiently emphasize the
importance of using the particular kind of chlorophyl which I mentioned.
In a brief communication to the editor of the _Photo. News_, in 1883, I
described some experiments with eosine as a color sensitizer, and then
called attention to the superiority of blue-myrtle chlorophyl for this
purpose, stating that I had not been able to secure such results with any
other kind of chlorophyl, and that a fresh solution from fresh leaves
must be used to secure the greatest possible degree of sensitiveness. See
_Photo. News_, Nov. 1883, p. 747.]
So far as I know, nobody tried the process. Nearly five years later Dr.
Vogel announced that, after eleven years of investigation, he had at last
realized a successful process of this character, and that this new
process of his was the "solution of a problem that had long been
encompassed with difficulty." This publication attracted a great deal of
attention, and gave me occasion to again call attention to my process,[3]
and point out that it was not only the first practical solution of this
problem, but the only truly isochromatic process ever discovered. Dr.
Vogel's new process was not only no better in any respect, but the plates
were insensitive to scarlet and ruby-red, and therefore would not
photograph all colors in the true proportion of their brightness.
[Footnote 3: _Photo. News_, London, September 5, 1884, p. 566, and _Year
Book of Photography_ for 1885, p. 111.]
My method consists in treating ordinary collodio-bromide emulsion plates
with blue myrtle chlorophyl solution, exposing them through the yellow
screen, and then developing them in the usual manner. The emulsion which
I have employed is made with an excess of nitrate of silver, which is
afterward neutralized by the addition of chloride of cobalt; it is known
as Newton's emulsion. I now prepare the chlorophyl from fresh blue myrtle
leaves, by cutting them up fine, covering with pure alcohol, and heating
moderately hot; the leaves are left in the solution, and some zinc powder
is added, which helps to keep the chlorophyl from spoiling. I have a
bottle of this solution which was prepared about six months ago, and now
appears to be as good as when first made.[4] A glass plate is flowed with
the emulsion, and as soon as it has set, the chlorophyl solution is
applied for a few seconds, after which the plate is washed in pure water
until smooth, when it is ready for exposure.
[Footnote 4: I originally recommended chlorophyl extracted from dried
leaves, because I had not yet learned how to preserve the solution for
more than a few weeks; and at some seasons it would be difficult, if not
impossible, to obtain fresh leaves. The tea organifier which I
recommended is also a color sensitizer, and when it is used in connection
with the chlorophyl from dried leaves the plates are as sensitive to red
as can be safely prepared and developed in the light of an ordinary
photographic "dark-room." Plates prepared with chlorophyl from fresh
leaves do not require treatment with the tea organifier to secure this
degree of sensitiveness. Recently I have used the tea organifier and some
other sensitizers, in connection with the solution from _fresh_
myrtle-leaves, and in this way have produced plates having such an
exalted color sensitiveness as to be unmanageable in ordinary "dark-room"
light. Possibly, such plates might be prepared and developed in total
darkness, by the aid of suitable mechanical contrivances, but I am not
sure that they would work clear even then, because they appear to be
sensitive to heat as well as to light.]
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.
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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