Thank you redgreenblue for suggesting me that excellent book.

E. J. WALL, F.C.S., F.R.P.S.
Author of
"The Dictionary of Photography,"
" Carbon Printing," etc.

Entered at Stationers' Hall
Electrotyped and printed, June, 1922

TO explain the action of tri-color filters it is necessary
to recall the statement that all colors can be
formed from three fundamental colors. But while we
have seen that these three fundamental colors give rise
to the three color sensations, it must not be thought
that there is any closer connection between the sensation
curves, shown in Fig. 3, and color photography.
The color sensation curves are quite useless for color
photography, although this is actually founded on their
physiological action, except when we use the additive
process, when the illumination of the three positives
should be made by the physiologically correct or fundamental
colors. But for subtractive work, and for the
making of the negatives for the additive processes, the
color sensation curves are not of the slightest practical
The first essential is to split up the spectrum or any
colored subject into three regions; the one representing
all the red, another all the green, and the third all the
blue. It has been found that the most satisfactory
results are obtained when the spectrum is divided into
three zones, shown in Fig. 6, in which R represents the
region transmitted by the red filter, G that of the green
filter and B that of the blue-violet filter. A comparison
of the curves with the color sensation curves in Fig. 3 will
at once show that there is but little similarity; in the
former the curves are gradual and all three overlap one
another at many points, whereas with the niters they
cannot be legitimately called curves, but are abruptly
400 460 500 560 590 700
FIG. 6
ending regions with limited overlap at only two points,
hi the golden-yellow, at the D lines, from 5900 to 5800,
and in the blue-green from 5000 to 4800.
If we separate these three curves we shall obtain a
fairly clear idea of the action of the filters. In Fig. 7, R
FIG. 7
shows the action of the red and the blank portion is that
which corresponds to the shadows of the negative. In G
the action of the green filter is shown, again with the
blanks representing the shadows, and B represents the
blue with the blank shadows. Now as it is the shadows
of the negatives that print, the bare parts of the above
diagram represent the parts that would print, therefore,
positives or prints from the above would be represented
by Fig. 8, in which the black parts show the parts that
FIG. 8
must not show in the additive process, that is when we
project by colored lights; while in subtractive processes,
that is in prints, they represent the colors that must be
present. We may, therefore, tabulate these results as
It will thus be seen that one may assume that the subtractive
printing colors are the complementaries of the
filter colors, and for this reason the corresponding negatives
are frequently referred to as the "minus color"
plates, as shown in the fourth column.
As the limits of the transmission of the niters are set
by the above statement, we have merely to make filters
that will show these cuts. The most satisfactory method
is by the use of aniline dyes, dissolved in gelatine and
cemented between glasses to protect the stained film
from damp and mechanical injury, for every photographer
knows that gelatine readily absorbs moisture,
and that it is easily damaged by dirty or damp fingers.
In the early days of color photography it was usual to
employ glass-sided cells filled with colored solutions, and
they are still employed sometimes in commercial establishments,
such as photo-mechanical studios. But for
the dilettante they are the most unsatisfactory form of
filter that can be used, and also when of sufficiently good
quality to allow large aperture lenses to be used, are
very costly, so that we shall not take them into consideration
at all. The preparation of the correct filters
does not present insurmountable difficulties for the average
worker, and it should be looked upon as part of the
preliminary training for every worker to make a set of
filters. Yet it must not be overlooked that it is actually
much more expensive to make them and the results are
rarely as satisfactory as the commercial filters. However,
we will proceed to describe the preparation of a set.
The apparatus required will be a sheet of plate glass,
not less than lox 12 inches in size and about half an
inch thick; three leveling screws and a level; some
sheets of plate glass about one-fourth inch thick and
sufficiently large to allow a margin of at least one-fourth
inch all round the largest filter that is required; a beaker
of 500 ccm capacity, another of 100 ccm, and an accurate
10 ccm pipette. The materials required are some
soft photographic gelatine, of which about 100 g will
be ample, and small quantities of the following dyes;
rose Bengal, tartrazin, patent blue, acid rhodamin, toluidin
blue and naphthol green. About 10 g of each will
be plenty. We also need some squares of patent white
plate glass, about one-sixteenth inch thick and of the
necessary size to cover the lens completely without cutting
off any of the light from the plate, and some
Canada balsam, dissolved in xylol, as used by microscopists,
which can be obtained commercially; it should
be noted that a chloroform solution of the balsam must
not be used.
The large sheet of glass is merely used as a support
for the gelatine-coated glass, so that the film shall set
of an even thickness all over, and therefore, its actual
size is not of real moment. It is frequently possible
to pick up a fairly large piece from a glass dealer
at a very reasonable price, because scratches and flaws
will not matter. The smaller pieces of glass, one-quarter
inch thick, are used to coat the dyed gelatine on,
while the one-sixteenth inch pieces are used for the final
filter. Both should have perfect surfaces. The reader
may possibly wonder why the dyed gelatine cannot be
coated directly on its final support; if this were done,
the chances are that the filter would not be flat, for the
gelatine contracts in drying and distorts the glass, that
is bends it into a shallow curve, and the result would be
that the filter would slightly alter the focus of the lens;
as this filter curvature might not be equal in all three
filters, the foci for the three colored images would differ
and the images would be of unequal size, and possibly
not equally sharp, and as we have to accurately superimpose
the constituent positives, the outlines of the subject
would not coincide and a confused jumble would be
the result, or we would get color fringes.
For this reason also, it is important that the final filter
glass be flat, and this is one of the difficulties in making
filters, that is, to obtain six pieces of glass that are sufficiently
flat to be serviceable. To test the glass for flatness,
place it on a flat black support such as a focusing
cloth or piece of velvet, this being placed at an angle of
45 degrees on a table about six feet from a window;
then, on looking down on it, the image of the cross bars
of the window will be seen reflected and usually a double
image, one from the front and a fainter one from the
back of the glass. If the glass or the eye be now moved
so that the images are reflected from all over the surface,
we can easily see whether the two surfaces are parallel,
as then the two images will keep the same distance
apart, whereas if lenticular they will either diverge or
converge. Pieces showing this defect should be rejected.
If the images are not straight, the glass is also curved.
It has been assumed that the glass is bought cut to the
required size; if it is bought in a large sheet it can be
examined in the same way and the flat parts marked
with a piece of soap and subsequently cut out; but
this presumes knowledge of how to cut glass with a diamond
or wheel. This, like everything else, is extremely
easy when you know the trick, but it will probably be
better to purchase the glass ready cut and select the
good pieces.
The size of the filter is easily determined, and a rough
and ready way is to measure the diameter of the lens
hood, not the glass, and aUow one-quarter inch beyond
this; this is assuming that the filter is to be used on the
lensi as it will be seen later that there are other positions.
A more correct method is to use a diagram such as is
shown in Fig. 9, in which L represents the lens racked
out from the plate PP, to its equivalent focus; then, by
drawing a line from the corner of the diagonal of the
Fig. 9
plate through the lens, the size of the filter can be at
once determined by actual measurement, as the distance
between the line from the corner of the plate to the
straight line A which is a prolongation of the optical
axis of the lens will give at once half of the necessary
width of the filter. Obviously the further the filter is
from the lens the larger it must be if it is not to curtail
the field or the light.
The gelatine solution is prepared as follows: Distilled
water must be used and it is well to make up a generous
quantity of the solution, say about 400 ccm, as it is
decidedly better to waste some than to run short. As
a 10 per cent solution is required, we weigh out 40 g of
gelatine, place in the 500 ccm beaker, cover with distilled
water and stir it well for two or three minutes, then pour
the water off and repeat the washing, allowing the
gelatine to soak for ten minutes the second time and
fifteen minutes the third, stirring it occasionally. Then
as much water as possible should be pressed out by
means of a glass rod and the beaker placed in hot water
at 55 C. (130 F.), when the gelatine will gradually
melt in the water that it has absorbed and enough more
is added to make the bulk up to 400 ccm. The solution
should now be filtered through a double thickness of
well washed and wetted linen; an old handkerchief does
well for this.
The dye solutions are preferably prepared in the form
of stock solutions as follows: i g to 100 ccm for rose
Bengal, patent blue, acid rhodamin and naphthol green;
2 g to 100 ccm for tartrazin and 0.5 g to 100 ccm for
toluidin blue. These quantities should be placed in
clean, well-dried bottles, 100 ccm of hot distilled water
added, the bottles well shaken for half an hour and
then allowed to stand so that the solutions may settle.
The quantities of the dye (not solutions) needed for the
average size of filter are very small and, therefore, the
amounts for a square meter are given, from which it
will be easy to calculate that for any given size:
For the red filter:
Rose Bengal 1.25 g
Tartrazin 2.0 g
For the green filter:
Naphthol green 0.4 g
Patent blue 0.2 g
Tartrazin i.o g
For the blue filter:
Acid rhodamin 0.6 g
Toluidin blue 1.8 g
The usual quantity of gelatine solution employed is 700
ccm per square meter, which gives a dry filter thickness
of about 0.4 mm; it is advisable to adhere always to a
given volume of dyed gelatine for a given area.
We can now prepare the glass. This should be placed
in a mixture prepared as follows. Make the following
Potassium bichromate 50 g
Water 250 ccm
Then add gradually:
Sulphuric acid 25 ccm
Stir well and add:
Water to make 1000 ccm
Fill a developing tray with this solution and immerse
the glasses, using a flat strip of wood to lift them, as
the solution bites the skin. Really the best way to clean
the glasses is to immerse one at a time and scrub the
surface with an old tooth brush or a temporary mop,
made by tying some old rags round the end of a short
stick, then turn the glass over and scrub the other side,
lift out with a stick and drop it into hot water, then
start cleaning another glass and remove the first from
the hot water, give it a rinse in distilled water and put
in a rack to dry. Then put the second glass in the hot
water and finish all in this way. It is possible to put
six or more glasses hito the cleaning liquid at one time,
but in doing so there is a very great chance of scratching
one or more of the glasses with a sharp edge or corner,
and although cleaning them singly is more trouble and
takes more time, it pays in the end.
When the glasses are dry they should be carefully
polished with a clean rag. These glasses are used for
the first coating and the dried dyed film has to be
stripped from them, but unless they are perfectly clean
it will not peel, and even then it is not always an easy
matter. All sorts of dodges have been suggested, such as
collodionizing the glass, waxing it, or polishing with talc;
but we may be content with a very simple plan. Add
about ten drops of almond or olive oil to 100 ccm of
benzol, pour five or six drops of this on the glass and rub
over with a tuft of absorbent cotton, and then polish
with two pieces of clean dry linen. An old handkerchief
again comes in handy, as it is usually fluffless; this will
clean the surface and yet leave the merest trace of oil,
which makes it easy to strip afterwards.
If the size of the finished filters were to be 5 x 5 cm,
we would naturally choose the first glasses 12 x 12 cm,
so as to get four filters from each sheet, which gives a
chance to pick and choose. To cover 144 qcm at the
rate of 700 ccm per square meter, we shall require 10
ccm of dyed gelatine and very small quantities of the
dyes, for instance in the case of rose Bengal 0.018 g and
0.028 g tartrazin, which means 1.8 ccm of the first solution
and 1.4 ccm of the second. While it is possible to
measure these quantities with a good pipette, it is preferable
to make at least 50 ccm of the dyed gelatine;
we, therefore, measure 34 ccm of the plain gelatine
solution, mix 9 ccm of the rose Bengal solution and 7
ccm of the tartrazin, and add these to the gelatine solution;
after stirring well, 10 ccm should be coated on the
Every worker has his own particular method of working,
and the author is addicted to the use of the pipette
for coating, as it has been found easier to lead the gelatine
over the surface of the glass with this than by pouring
out of a small graduate, and there is less chance of
bubbles. If these do occur they can very easily be led to
the edge of the glass, where they will do no harm, and
can be broken by touching with a spill of blotting or
filter paper.
It is important that the room in which the coating is
done be not too cold or the gelatine will set before it is
possible to spread it over the glass, and the stock of
dyed gelatine should be kept at a proper temperature.
It is easy to do this with a water bath, and 45 C.
(123 F.) is a comfortable temperature to work at, as
the gelatine can then be easily spread, and it sets rather
rapidly to an even surface. As soon as thoroughly set,
the coated glass should be reared up on edge to dry
and the method suggested for drying sensitized plates
may be adopted, but dust must be carefully guarded
against, and before commencing to coat, the working
bench should be well washed down with water so as to
ensure that no dust lies about. Rapid drying is not of
such moment with niters as with sensitized plates, but it
should be as even as possible; if it takes too long, the
film may become pitted with small colonies of spots due
to bacterial growth, particularly in hot weather.
When the gelatine is perfectly dry, it should be cut all
round the edges, about 2 mm inside, with a sharp penknife,
and usually it is possible to lift one edge with the
knife and, by taking hold of the free edge, to strip the
whole film; if not, it should be held for four or five minutes
about two feet above a bowl of steaming water,
when it should strip without trouble and without stretching.
As soon as stripped, the film should be placed between
tissue paper and stored between the leaves of a
book. The used glasses can be freed from the narrow
strip of dyed gelatine by scraping with the knife, repolished,
and again coated with the second lot of dyed
gelatine, but it is preferable to have at least six glasses
and coat them one after the other, thus making one job
of it. Naturally the pipette will be well scoured out
with hot water.
It will be seen that the process is not such a difficult
one and with a little practice it is easy to become so
expert that a failure is rarely met with. But for years
the author has given up coating his own filters, except
for experimental purposes, and is content to buy commercial
filters. The results are absolutely certain and the
actual cost is so very little, that taking into consideration
the difficulty of getting good glass and the fact
that one has to buy much larger quantities of dyes than
one needs, commercial filters actually work out cheaper
in the end.
Commercial filters can be obtained in the form of film
or cemented between glasses. Assuming that the worker
has made his own filters or bought commercial film
filters, we come to the operation of cementing them,
which is not nice. The final glass must be carefully
cleaned and the polished surfaces should be placed in
contact, pair by pair. We now want to fasten these
together like a book cover, and use for this purpose a
piece of lantern-slide binding, making quite sure that
the edges are even. This can easily be done by placing
the wetted strip on a yielding surface, such as an open
book; then, holding the two glasses together, lower them
on to the strip with an equal margin on each side,
press down, and rub the edges of the strip into contact
with the sides of the glass. There is no need to let this
get absolutely dry, but it should not be wet. The dyed
film, cut to the same size as the glass, is slipped between
the two glasses and all edges made to coincide.
The filter should now be laid down on a sheet of hard
card or paper, the top glass and the gelatine film
lifted up together, and a pool of balsam poured on the
lower glass near the hinge. Enough balsam should be
used to cover about one-fourth of the surface of the
glass. The gelatine film should then be carefully lowered
down on the balsam, another pool of balsam poured on
the gelatine and then the top glass lowered. Gentle
pressure, starting from the hinge, will force the balsam
out to all the edges without the least air bubble. If one
does show, it can be chased to the edge by pressure of
the fingers. A good sized metal bulldog clip should now
be clipped on the glass at right angles to the hinge, as
far in as it will go and then another applied opposite the
first; the paper hinge should be scraped off the edge
with a knife, paying no attention to that on the flat of
the glass, and a clip placed here and then one on the
fourth side. The filter may then be reared up on one
corner to allow the balsam squeezed out from its edges
to run down. When all the filters are cemented, begin
with the first, wipe off the exuded balsam with filter
paper or a bit of old rag, and leave the filters to dry.
They really require a warm place and should be left
three weeks in a flat position, so that as soon as the first
lot of exuded balsam has been wiped off they should be
placed flat on a glass or shelf with a piece of paper
underneath them, as the balsam that exudes will cement
them to any thing that they touch. Balsam is one of
the stickiest of all sticky substances and the best way
to clean the fingers is to well wet newspaper with denatured
alcohol and rub well, using as much clean paper
as possible and then benzol and paper, followed by
plenty of soap and hot water.