While directions have been given for making a filter, it must be understood that this is for the autochrome plate. While it can be used for the Paget plate, the makers of this issue special filters and it is well to purchase these. These filters are adjusted for average daylight, and they cannot be used successfully with any other light, so that special filters must be prepared for these. It may not seem worth while to make these filters, but possession of a set enables winter and evening work to be attempted. The following formulas are, therefore, given to complete the information.

For Use With Nernst Lamps

Gelatine, 1: 15 solution 40 ccm.

Tartrazin, 1: 2,500 solution 3 ccm.

To this add:

Aesculin 0.4 g.

Distilled water 37 ccm.

Ammonia 3 drops.

To every 100 square cm allow 8 ccm of the above solution. The aesculin solution must be made only just before use. This filter is combined with a blue filter of the following composition:

Patent blue, 1: 1000 solution 2 ccm.

Gelatine, 1:15 solution 46 ccm.

Distilled water 38 ccm.

Allow 7 ccm per 100 square cm.

For incandescent gas, the same filters are used, but the quantity of the blue gelatine is reduced from 7 to 5 ccm for the same area.

For a 25 ampere arc:

Gelatine, 1:15 solution 40 ccm.

Tartrazin, 1: 500 solution 4 ccm.

Pheno-safranin, 1: 7000 solution 1 ccm.

Add to this:

Aesculin 0.4 g.

Distilled water 35 ccm.

Ammonia 3 drops.

Allow 8 ccm for the same area. This filter is particularly useful for photo-micrography with screen-plates.

For flashlight work, the composition of the filter will depend on that of the flash mixture; Lumiere's Perchlora mixture is two parts of magnesium powder and one part of potassium perchlorate, and for this the correct filter is:

Gelatine, 1:10 solution 40 ccm.

Tartrazin, 1: 500 solution 5 ccm.

Pheno-safranin 1: 7000 solution 3 ccm.

To this add:

Aesculin 0.4 g.

Distilled water . 32 ccm.

Ammonia 4 drops.

Allow 8 ccm per 100 square cm.

F. Novak suggested a mixture of two parts of magnesium and one part of dry thorium nitrate, and the compensating filter for this is:

Gelatine, 1: 15 solution 100 ccm.

Filter yellow, 1: 200 solution 15 ccm.

Crystal ponceau, 1: 800 solution 4 ccm.

Water 4 ccm.

Allow 7 ccm per 100 square cm.

Exposure Of The Screen-Plates

In ordinary black and white photography it is well known that there is a great latitude in exposure. One may make an error of two or three times one way or the other and yet obtain a negative that will give a good result; but with screen-plates this is not possible. A really correct rendering of the colors is only possible with correct exposure, this being due to the extreme thinness of the film of emulsion.

Various tables have been published as guides for the novice, but these are not reproduced for the simple reason that the factors involved in estimating the correct exposure are so delicate that they cannot be determined from a table with a sufficient degree of accuracy. As a standard cf exposure, the makers of the autochrome plate state that one second with a lens working at /:8 in summer and sunlight near the middle of the day is correct; but everyone knows that the intensity of sunlight may vary enormously. Then again, one may wish to take an interior, or it may be necessary even in sunshine to use a very small stop; theoretically one has merely to increase the exposure in direct ratio to the decrease in illumination, so that in the case of the diaphragms, assuming that one used f: 32 instead of f: 8, the increase should be as 1:16, whereas actually it would be more like 1: 24 or 1:32. Exactly in the same way, when working in a comparatively feeble light, the increase in exposure does not follow the usual law that intensity of illumination and duration of exposure are inversely proportional. This failure is well known, but is not often of importance in black and white work, but with screen-plates, in consequence of the very low practical sensitiveness of the emulsion, it has to be considered. The expression "practical sensitiveness" is advisedly used, for that of the autochrome emulsion is about 36 H. & D., but the color elements cut this down to three and the filter to one and one half.

The most satisfactory method, in fact, the author goes so far as to say, the only satisfactory method of determining exposures with screen-plates is to use one of those meters which measure the intensity of the light by the darkening of a strip of sensitive paper. The two most useful meters of this type are those manufactured by Watkins and Wynne, in England. In these special paper is employed, actually a specially treated bromide and not a printing-out paper, and the instruments are provided with special scales that compensate for the failure of the above law. If tables are useless, so also are those meters that gauge the light by visual examination or reduction by absorbing material.

When the separate method is used, one is not tied down to the use of such slow emulsions, and may use the fastest panchromatic plates obtainable; in this case the usual speed must be adopted for use with the meters and the increase necessitated by the taking screen worked out. It has frequently been recommended to hypersensitize autochrome plates by bathing them in various dye solutions, such as pinachrome, etc.; but this is not a process that should be attempted by the novice. A special filter is required and the result is rarely correct.

It may be as well to call attention again to the fact that the screen-plate must be placed the wrong way round in the dark slide, that is, with the glass toward the lens. It is essential, therefore, that the emulsion be protected from damage by any spring which keeps the plate in register. The autochrome plate is issued with a piece of black card in contact with the film, the other side being white, and this should be put in the plate holder, back of the plate. For the separate method it is imperative that the taking screen and the sensitive emulsion be in as close contact as possible, and it will be found that the English, or book-form dark slide, is much superior to the average American plate holder, not only in ease of filling, but also in giving closer contact. Finally, the back of the glass must be clean; any dirt or finger marks will be visible in the picture as a darker patch. It is easy to clean the glass after loading in the dark room, by pulling out the sliding shutter and polishing with a clean cloth.