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    Mustafa Umut Sarac's Avatar
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    Tri-color Filters Making Tutorial from Early 20th Century - 1922

    Thank you redgreenblue for suggesting me that excellent book.

    https://archive.org/details/practicalcolorph00walliala

    PRACTICAL COLOR
    PHOTOGRAPHY
    BY
    E. J. WALL, F.C.S., F.R.P.S.
    Author of
    "The Dictionary of Photography,"
    " Carbon Printing," etc.
    SOUTHERN
    AMERICAN PHOTOGRAPHIC PUBLISHING CO.
    BOSTON 17, MASSACHUSETTS


    1922
    COPYRIGHT, 1922, BY
    AMERICAN PHOTOGRAPHIC
    PUBLISHING CO.
    Entered at Stationers' Hall
    PRINTED IN THE UNITED STATES OF AMERICA
    Electrotyped and printed, June, 1922
    THE PLIMPTON PRESS
    NORWOOD, MASS. U.S.


    COLOR FILTERS
    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
    value.
    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
    B G R
    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
    follows:
    Filter
    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
    solution:
    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.

  2. #2
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    An alternative method is to leave the hinge on, place
    the filter on a thickness or two of blotting paper on a
    level surface and place a card on top with a good-sized
    weight, four pounds not being too heavy for a 12x12
    cm filter. Direct heat cannot be used for drying, as this
    causes the edges to dry first and gives rise to distortion.
    At the end of the three weeks the exuded balsam
    should be scraped off, and the glass cleaned with alcohol
    and newspaper, then with bits of cloth and finally
    polished. Do not try to be sparing with the cleaning
    cloth, or use one large cloth; little bits and each piece
    thrown away as soon as it gets sticky is the easiest way.
    The final polishing should be done with tissue paper and
    alcohol, following the same plan, that is, fresh pieces
    continually. Benzol, xylol or chloroform should not be
    used, as they are energetic solvents of balsam and will
    almost inevitably creep in between the edges, in which
    case the job will have to be done all over again.
    Those who would like to make their own preparation
    of balsam may purchase some dried Canada balsam from
    a lens worker or optician. This should be roughly
    powdered, which is most easily done, though it is rather
    wasteful, by tying it up in a cloth and hammering it
    with a heavy hammer; a fine powder is not wanted but
    the big pieces should merely be broken up. Then place
    this in a wide mouthed bottle, place in the water bath
    and bring the latter slowly to a boil, stirring the balsam
    all the time; add about one fifth of its weight of xylol,
    stirring well and then letting it get cold in the water
    bath. This preparation requires a much higher temperature
    to melt and must be used hot. It then sets very
    quickly and at a pinch a filter thus cemented may be
    used the next day. The only difficulty likely to be met
    with is the setting of the balsam before an even film is
    obtained, but warming the glasses, or keeping them on
    a hot plate for some time under pressure will soon make
    the balsam spread out.
    There are four possible positions for the filter; in
    front of the lens; between the combinations close to the
    diaphragm; behind the lens; and immediately in contact
    with the sensitive surface. Between the lenses is
    the very worst place to choose, although this requires
    the smallest filter, as is obvious. In the first place, it
    is very likely to upset the corrections of the lens, particularly
    with the later forms of anastigmatic lenses, and
    with these there is often not enough room to insert any
    other than a film filter. Secondly, it is not easy to
    change the filters in this position without some special
    fitting, so that we can dismiss this at once. Either in
    front of or behind the lens may be chosen, which one
    being a matter of indifference, provided focusing is
    always effected through the filter, a matter that we
    shall have to deal with later on when talking of screen
    plates (See Chapter XII). In either case some sort of
    sliding fitting is advisable, although this is not conve
    lent in some cases inside the camera, as not only may
    the rear lens protrude beyond the lens board, but one
    has to have some means of shifting the filter between
    exposures, which necessitates a light-tight fitting.
    It is possible in many cases to arrange a frame to
    slide over the camera front, and to fit the ordinary lens
    panel on this, so that the filters will be behind the lens.
    The sliding frame can be made on the same lines as the
    usual lantern slide carrier, and if velvet is used to line
    the outer frames there will be no trouble in making it
    light-tight. Or it may be possible to fit such a frame on
    the lens barrel itself, but here it must be so securely
    fastened that there is no chance of its slipping off.
    Really the simplest plan is to obtain one of the square
    slip-on cells, which, fitting on the lens hood or barrel,
    may be always retained in position and the filters merely
    lifted out and inserted as required. It is advisable, if
    possible, to remove the lens hood and fit the holder on
    the barrel, as this means not only slight reduction in
    size, but as a rule a firmer hold. In order to obtain the
    correct size of fitting, the diameter of the lens tube
    should be taken with a pair of sliding calipers. Failing
    these, the next best plan is to take a narrow strip of hard
    writing paper and wrap round the lens barrel so that the
    ends overlap by about half an inch, then with a sharp
    penknife cut right through both pieces of paper midway
    of the overlap, not at the end.
    Placing the filter close to the plate means that the
    filter must be of the same size as the plate. Defects in
    the filter, such as want of absolute parallelism of the
    surfaces, are here of the least consequence; but local
    defects, such as coating striae or bubbles, are more
    apparent on the negative image, though only locally.
    Special sliding backs can be? obtained commercially, fitted
    with the three niters and made to take three plateholders,
    or with some plate-holders, particularly of the
    English book-form pattern, the filter may be placed in
    actual contact with the sensitive surface; then naturally
    its thickness must be allowed for in focusing.
    One important point in the choice of filter fittings,
    particularly metal ones, is that there should be no
    abnormal pressure on the glasses, as this may cause
    strain and consequent degradation of definition. It
    should be possible to turn the filter round, or shift it,
    with the lightest pressure of the fingers. Neither is it
    advisable to use cells screwing into the lens hood, as
    this is almost certain to shake the camera and there
    is much loss of time in changing.
    For photomechanical work, in which long-focus lenses
    are nearly always used with half-tone screens, the glass
    must be optically worked, as carefully, in fact, as the
    lenses themselves. Such glasses are known commercially
    as
    "
    optical flats," and are very costly if of any size.
    They must all be absolutely the same thickness and be
    so arranged that they are always perpendicular to the
    axis of the lens.
    The ordinary filters may be used for making the separation
    negatives for photomechanical work, as if there is
    not absolute coincidence of size, this can be corrected by
    the operator when making the screen negatives from the
    transparencies, though he will not be pleased at having
    to do this.
    It may possibly be as well to interpolate here a note
    as to the making of the constituent negatives for photomechanical
    purposes. The use of the panchromatic
    gelatine plate for this work is largely on the increase,
    and in some cases the slow panchromatic plate is used
    for making the color separation and the screen negatives
    in one; but the usual practice is to make the separation
    negatives first, from these a set of transparencies, and
    then the screen negatives. It may be noted that the
    transparencies for such work should be as little like a
    lantern slide as possible. They should be fully exposed,
    quite "soft" in character, and with practically no bare
    glass except in the very deepest shadows. Full exposure
    should be given to the plates, and it is better to use slow
    negative rather than transparency plates for this work,
    as giving a longer range of gradation and less tendency
    to brilliancy. The exposure should be full and development
    not pushed too far, so that the highest densities
    are quite transparent.
    In many commercial process establishments collodion
    emulsion, and even the wet plate process, still hold their
    own for the making of the separation negatives, and also
    the combined separation-screen negatives. Usually the
    emulsion is obtained commercially with its special sensitizers,
    and the makers issue instructions for the making
    of the filters, which are usually of the liquid cell type,
    for use with the same. On the other hand the method of
    sensitizing already advised may be adopted, or the dye
    may be added to the enrulsion, and in this case 80 ccm
    of sensitol violet stock solution should be added to 1000
    ccm of the plain emulsion, and the plates washed in running
    water or under a rose tap for fifteen minutes. The
    washing increases the sensitiveness of the plates about
    five times.
    By some writers it has been proposed to use different
    plates for the different color separations, that is to say,
    an ordinary, non-color-sensitive plate for the minus
    yellow negative; an orthochromatic plate for the minus
    red negative; a panchromatic or red-sensitive one for
    the minus blue negative. This plan may at first sight
    appear to have certain advantages, but this method is
    not one that should be adopted. It is a well established
    fact that the degree of contrast, or gamma, differs
    with different kinds of plates, and in fact with different
    batches of the same kind of plate, to say nothing of the
    development velocity of the plates, and one of the most
    important essentials in making separation negatives is to
    have them of the same degree of contrast as far as possible.
    That is to say, in the three negatives the range
    of densities of a black and white scale should be the
    same; and with three totally different kinds of plates
    this is almost an impossibility. One kind of plate
    should be used for all three separation negatives, and
    they should be, as already pointed out, as far as convenient,
    developed together. The adoption of this plan
    will save no end of after manipulation and dodging in
    getting concordant results; and it may be taken as an
    axiom that hand work, except for the removal of purely
    mechanical defects, such as pinholes, etc., cannot be successfully
    executed with color negatives.



 

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