The adjustment of the filter absorptions is rather a delicate matter; but a suitable filter for A can be made from rose Bengal 0.8 g per square meter, or phenosafra-nin 0.48 g, or a Wratten & Wainwright No. 31, minus green 1, may be used. The G filter should be the normal tri-color green filter, as previously advised. O may be made with tartrazin 4 g, or naphthol orange 3 g per square meter; or a Wratten & Wrainwright No. 21, monobromofluorescein, may be used. For B a Wratten No. 49b, C 4 dark, should be used. Probably this filter could be matched with crystal violet 1 g and patent blue 1.6 g per square meter; but as these dyes cannot be mixed they must be coated on separate glasses and cemented together. Methylene blue 0.8 g, or toluidin blue 2 g, might take the place of the patent blue; but these do not absorb as much violet as patent blue, which has a marked absorption band commencing at G 1/2 H and extending into the ultra-violet.

The Camera And Exposure PracticalColorPhotography 12

Fig. 11.

In this, as in the other camera, the optical paths through glass must be equal, therefore, filter G must be 1.41 times the thickness of A plus 1.41 times the thickness of B, and filter O must be 1.41 times the thickness of A. A compensating filter must be used on the lens to equalize exposures and probably that suggested for autochrome plates on p. 143 will be very close to the ideal.

One advantage of this arrangement of filters is that it gives the yellow image direct from P3, and this forms the basis for the constituent pictures, which is advantageous, as opaque yellow pigments may be used for prints. The special advantage of the dark blue filter is that it cuts out some of the violet and therefore, gives a more brilliant negative for the yellow. Naturally the same arrangement of filters as advised for the previously described camera may be adopted.

The distances of the filters from their respective plates must be controlled in the same way, that is, by making negatives of geometrical figures and superposing them. An expansion of the figures in one negative will show that it is too far from the plate, and vice versa.

The simplest form of camera is undoubtedly the semi-dialyte. This is dealt with elsewhere (see p. 222), and also the objections to this system.

With regard to the lens that should be used for color work, no special instructions need be given, as any lens may be used, but some are more suitable than others. The newer anastigmats are preferable, as they cover sharply a given size plate at a larger aperture than the older types, and this is a consideration in consequence of the increase of exposure incident to the use of filters.

Every plate maker gives, as a rule, the ratio of exposures for his plates with a given set of filters; but this can always be determined by experiment and the best method is to use a scale of greys, made by exposing a piece of bromide paper, in geometrical ratios, to white light and then developing. It is only necessary to place the edge of the bromide paper between the edges of a book, and expose to white light for say one second, then push the paper in about half an inch and give another second exposure, then push the paper further in and give two seconds exposure, doubling the exposure in each case so that the result will actually be as follows:

First exposure 1 second

=

1

Second exposure 1 second + (1 already given)

=

2

Third exposure 2 + (1 + 1)

=

4

Fourth exposure 4 + (2 + 1 + 1)

=

8

Fifth exposure 8+(4 +2+1+1)

=

16

Sixth exposure 16 + (8 + 4 + 2 +1 +1)

=

32

This should give six steps of varying density, which will be quite sufficient to determine the filter ratios, for one has merely to make three exposures through the three filters with this graduated strip as the object, to obtain three negatives. A comparison of the images will give one a very good idea whether the ratios adopted are correct or not, for if they are, the scale of greys will be alike in all three negatives. If one strip of the bromide paper is shielded from all light, and the other end quite black, it is easy to determine how the exposure ratios must be altered to attain equality.

Theoretically, one ought to determine the filter ratios before each exposure, as the color composition of daylight varies considerably, being very much richer in red and green in sunlight than in shadow or in cloudy weather. But if the filter ratios have been determined, one may ignore this factor, at any rate at first. The actual exposure required without a filter should be determined by means of one of the exposure meters that actually measure the chemical intensity of the light by the darkening of a paper to a standard tint. These are much more reliable than those in which the brightness of an object is determined by visual comparison with a standard; and all tables based on latitude and classification of subjects are misleading for color photography.

It is extremely difficult to tell from a set of negatives which filter was used for a particular negative. One might imagine that this would be an easy matter, but it is not, and even after long experience it is easy to be misled. It is, therefore, always well to make the exposure automatically record the filter. If the filters are used in contact with the sensitive surface this is very easy, for one has but to affix to the face of each filter, placed next to the sensitive plate, an opaque letter designating the color. When the filters are used on the lens, some little device should be thought out which can be permanently affixed to the plateholders and which will be automatically recorded by the light. Several of these will at once suggest themselves, such as distinct file cuts in one edge. The author has been in the habit of using small triangular pieces of metal driven into the corners of the front of the plateholders, one piece in one, two pieces in opposite corners of another, and none in the third. There is no difficulty in at once picking out the negative recording each color, provided the same holders are always used for the same filters, that is, the one without any corner piece for the blue, that with one for the green, and that with two for the red, or any other order that is decided upon. The corner pieces need not be very large; a right-angled triangular piece measuring about 5 mm from the apex to the base is quite large enough. This leads to the suggestion that order is a capital essential in color work, and that exposures should always be made with the filters in one particular order, which should be rigidly adhered to for all time, as then one does not have to think as to whether one filter or the other has been used. This particularly applies when separate exposure work is undertaken, and while it may be extremely nice to be able to tackle any and every subject that comes up from portraits to landscapes and bric-a-brac, yet some of the most valuable work may be done with an ordinary camera, three separate plateholders and three exposures, confining one's work to subjects that will not move, or which at least give one time enough for three separate exposures. As illustrative of this may be mentioned the case of a certain worker, who happened to see the author at work on some color prints and wanted to know how "the job was done." His photographic experience was probably like that of many another amateur; he had dabbled in black and white work and had shot at everything and anything, and then gotten rather tired of it because he had no purpose in his work. He decided to attempt color work, using his old camera, with three plateholders and three separate exposures, and his first filters as well as the plates were borrowed from the author. The only subject that he could find which was sufficiently brightly colored to satisfy him and would certainly not move, was a somewhat brilliantly colored old china vase. Curiously enough, possibly another example of beginner's luck, an exceedingly good result was obtained. Succeeding failures with landscapes induced our friend to "stick to his pots," as he expressed it, with the result that now he has the finest collection of color prints and slides of old china in the world and has become one of the leading experts in that field, as he was led to learn all that could be learned about his pots.