Prepare 7 solutions, 4 of which are used for color screens, the remaining 3 serving as dyes for the plates.

A.—Screen Solutions.—

Blue violet. by weight

Methylene blue.... 5 parts

Tetraethyldiamidooxytriphenyl carbinol........... 2 parts

Or By weight

Methyl violet...... 5 parts

Alcohol.......... 200 parts

Water, distilled.. .. 300 parts

Green. By weight

Malachite green... 10 parts

Alcohol.......... 200 parts

Water, distilled.. .. 300 parts

Yellow. By weight

Acridin yellow N. 0.............. 10 parts

Alcohol.......... 200 parts

Water, distilled.. .. 300 parts

Red. By weight

Congo rubin...... 10 parts

Alcohol.......... 200 parts

Water, distilled.. .. 300 parts

B

Dyes (Stock Solutions).—

By weight I

Acridin yellow or acridin orange,

N. 0........... 1 part

Alcohol.......... 100 parts

Water, distilled.. . . 400 parts

By weight

II

Congo rubin...... 1 part

Alcohol.......... 100 parts

Water, distilled.. . . 400 parts

By weight III

Tetraethyldiamidooxytriphenyl carbinol........... 1 part

Alcohol.......... 100 parts

Water, distilled.. .. 400 parts

The screen solutions, after being filtered through paper filters into clean dishes, are utilized to bathe 6 clean glass plates previously coated with 2 per cent raw collodion; we require 1 plate for blue violet, 2 plates for red, 2 plates for yellow, and 1 plate for green, which in order to obtain the screens are combined in the following way: Yellow and red plate, yellow and green plate. For special purposes the other red plate may be combined with the blue violet. Another method of preparing the screens is to add the saturated solutions drop by drop to a mixture of Canada balsam and 2 per cent castor oil and cement the glasses together. Those who consider the screens by the first method too transparent, coat the glass plates with a mixture of 2 to 3 per cent raw collodion and 1 per cent color solution. Others prefer gelatin screens, using

By weight Hard gelatin (Nelson's) .......... 8 parts

Water............ 100 parts

Absolute alcohol.. . 10 parts

Pigment.......... 1 part

This is poured over the carefully leveled and heated plate after having been filtered through flannel.

The collodion screens are cemented together by moistening the edges with Canada balsam (containing castor oil) and pressing the plates together in a printing frame, sometimes also binding the edges with strips of Japanese paper. On the evening before the day of work, good dry plates of about 18° to 24° W. are dyed in the following solution:

By weight

Stock solution, No. 1     16 parts

Distilled water......   100 parts

Alcohol............       5 parts

Nitrate of silver

(1.500)..........     50 parts

Ammonia..........   1-2 parts

This bath sensitizes almost uninterruptedly to line A. The total sensitiveness is high, and the plate develops cleanly and fine. Blue sensitiveness is very much reduced, and the blue screen is used for exposure. As far as the author's recollection goes, the plate for the yellow color has never been color-sensitized, many operators using the commercial Vogel-Obernetter eosin silver plates made by Perutz, of Munich; others again only use ordinary dry plates with a blue-violet screen. This is, however, a decided mistake, necessitating an immense amount of retouching, as otherwise it produces a green shade on differently colored objects of the print.

For the red color plate the dry plate is dyed in

By weight

Stock solution, No. 2 10 parts

Distilled water...... 100 parts

Nitrate of silver

(1.500).......... 100 parts

Ammonia.......... 2 parts

The resulting absorption band is closed until E, reaching from violet to red (over C). This red pigment was examined by Eder, who obtained very good results, using ammonia in the solution.

The corresponding screen is a combination of malachite green with acridin yellow or acridin orange N. O.

For the blue color plate the dye is made up as follows:

By weight Stock solution, No. 3 0.5-1 part

Distilled water...... 100 parts

Nitrate of silver (1.500).......... 100 parts

Ammonia.......... 1-2 parts

This dye yields a strong band, commencing at B, reaching to C J D; since the orange screen used herewith necessitates a long exposure, the action seems to extend into the infra-red (beyond A).

As a rule, cyanine is used instead of the tetraethyldiamidooxytriphenyl carbinol (HC1 salt), but the former is apt to produce fogged plates. Methyl violet or crystal violet has also been suggested.

Exposures should be made in direct sunlight or with artificial pure white light (acetylene); electric light is too variable.

The most suitable methods of reproduction are half-tone, and the prototype methods; also Turati's Isotypic The greatest difficulty in 3-color printing nowadays is presented by the want of accurate printing. We must use the proper paper and pure fast colors; the inking rollers should be smooth, not too soft, and free from pores or weals. The blocks must be firmly fixed typehigh, otherwise they take color irregularly. A good printing machine is, of course, most essential.

To supplement the above working directions: After having kept the plates for 2 or 3 minutes (constantly moving the dish) in the dyes, they are removed into a dish containing filtered alcohol, which extracts the superfluous pigment. Plates thus treated dry much more rapidly, develop cleaner, and show no fogging.

Most of the above dyes may be obtained from the "Berliner Actiengesellschaft für Anilinfabrikation," the acridin only from the "Farbwerk Mühlheim, a/Main, vorm. A. Leonhard & Company."