All the original formulas for color-sensitizing called for the old cyanin, but much better results can be obtained with the newer isocyanins, and pinachrom is very satisfactory. For the bath method use:

Pinachrom, 1:1000 alcoholic solution 5 ccm Distilled water to 1000 ccm.

Borax, saturated solution 10 ccm.

Bathe four minutes, rinse, and dry rapidly. As full details have already been given as to bathing and drying plates there is no need to add more. Green light must be used as soon as the dye is added, but before that a bright orange light may be used. If it is thought that sufficient red sensitiveness is not obtained with the above dye, though this will only be noticed in spectrophotog-raphy, then a mixture of sensitol green and sensitol violet may be used, in the ratio of two parts of the former to one of the latter; and not more than 6 ccm should be added to the bath. If the dye is to be added to the emulsion, then 3 ccm of the dye solutions should be added to 1000 ccm of emulsion.

Before leaving the emulsion making it may be useful to point out that H. E. Ives has stated that the best results are obtained in spectrum work, for which the process is eminently suited, by using the following solutions:

A. Gelatine 20 g.

Distilled water 500 ccm.

B. Gelatine 40 g.

Potassium bromide 5 g.

Distilled water 1000 ccm.

C. Silver nitrate 6 g.

Distilled water 100 ccm.

Dissolve the gelatine, cool, add the salts, then add A to C and pour into B. Working temperature 35 ° C. (95 ° F.). Coat the plates by flowing the emulsion on to cold glass, drain off and set, and wash the plates for fifteen minutes. The emulsion should be color-sensitized by adding 1 ccm of the dye solution to 100 ccm, or for bathing the strength should be 1: 100,000.

As previously stated, the film is exposed in contact with mercury, and obviously the glass must face toward the lens. The chief difficulty here is the plate-holder; it must be capable of holding the mercury without leakage, and the mercury must be run in and out when the plate is in position. Specially constructed holders are obtainable commercially and it will be better to purchase one; but the following brief description may be useful. The front of the holder carries a strip of rubber about 4 mm wide and 2 mm thick, against which the glass is pressed. The back is a solid piece of wood, metal lined, with a rubber strip all round, and this is a firm but resilient red rubber. The back fits tightly in place and is held by two steel cross bars that fit on pins with fly nuts which screw it firmly into place. In one corner of the back is screwed a small steel pipe to which is fastened a rubber tube, connected with the mercury reservoir; at the diagonally opposite top corner is a screw, with a hole driven right through its length and a small cap fitted; the purpose of this is to allow egress of the air as the mercury flows in. The lower pipe is fitted with a tap; to fill the slide with mercury, the tap is opened, the screw cap unloosened and the container slowly and gradually raised until it is higher than the top of the holder; the tap is then turned off and the container can be laid on the back of the camera. After exposure, the tap is opened, the container slowly lowered, and the mercury runs back. This holder and the mercury make the whole very heavy and allowance must be made for this.

It is very important that the mercury be kept clean, and if pure in the first place it can be used repeatedly without trouble. The best way to clean it is to procure a large chamois leather; well wash this in warm water and soap, and dry; then soak in gasoline for an hour or two, kneading it occasionally with the hands, and dry. This may make the leather harsh and stiff, but it can be easily suppled by pulling and working between the hands. It should be placed in a clean dish and the mercury poured into the middle and run backwards and forwards for a short time, and then the ends of the leather gathered together and twisted round so as to force the mercury through.

One caution is necessary; immediately after exposure the mercury must be run out; if left in contact with the film, the latent image on this is completely destroyed.

It is impossible to give a very definite idea of exposure, but the plates are from 1000 to 2000 times slower than a fast plate. For a landscape in bright sunlight, with a lens working at f: 4.5, the exposure will be from one to two minutes. It will be seen that portraiture by this process is almost impossible, though Professor Lippmann did succeed in taking some very good portraits. For still life the plates are excellent. For spectrographs work, with a slit width of 0.3 mm, a condenser and sunlight, the exposure will be about one minute.

For development pyro-ammonia has been the favorite, and the following is a typical formula:

A. Pyrogallol 1 g.

Distilled water 100 ccm.

B. Potassium bromide 20 g.

Ammonia, sp. gr. 0.96 67 ccm.

Distilled water 100 ccm.

For use, mix 10 parts A, 20 parts B, and 140 parts water. Duration of action about three minutes, and the plate should be at once well washed.

There has been considerable discussion as to whether these pictures should be fixed or not; for the most correct rendering of color they should not be, and this is particularly applicable to spectrum work. If they are fixed the distance of separation of the silver laminae is slightly reduced and consequently there is a slight change of color. For ordinary work, the best fixer is a five per cent solution of potassium cyanide, which should not be allowed to act longer than ten to fifteen seconds; then the plates should be rapidly washed under a tap for about ten minutes and dried.