The glass of which the screens are made should be selected patent plate, about in. thick, and it should be as parallel as possible ; for the best quality screens optically worked glass should alone be used. In order to test the glass for parallelism of its sides, it should be placed on a sheet of black velvet and held at an angle of about some distance from the cross-bars of a window, so that the reflection of these can be seen in the glass. On turning the glass round on the velvet, the image of the cross-bars will be seen to be doublethat is, the reflection from both the front and back of the glass will be seen. These double images should, as far as possible, be constant in position one to the other, and not shift up and down or from side to side. The glass must be thoroughly cleaned and placed on a thick sheet of plate glass which has been accurately levelled, and the necessary quantity of dyed gelatine poured on to it, coaxed out to the edges with a glass rod, and allowed to set, when it can be put away to dry.
It may be pointed out here that the position of the filter has some effect on the focal length of the lens and also on the definition of the image. When placed behind the lens it slightly lengthens the focus, but its effect on definition is a matter of actual test with every filter, though the nearer the filter is to the plate the less the effect ; therefore, obviously, if placed in contact with the sensitive surface, the ques tion of the quality of the glass is not of so much moment, and in this case even fixed-out and washed dry plates may be used.
The cementing of colour screens is a messy process, and requires considerable practice to perform successfully. A fairly thick solution of Canada balsam in xylol, such as is used by micro scopists, should be used, and the screens well warmed for at least half an hour, so as to ensure that they are thoroughly dry ; the balsam should also be gently warmed. A pool of balsam, about half the size of the plate, should be poured on to one screen somewhat near one edge, and the other screen lowered first on to this edge and slowly allowed to fall down, when it will squeeze the pool of balsam out so as to cover the whole surface. Then a stout bulldog clip should be fastened on each side, and the screens put away in a warm place for the balsam to dry out ; this will probably take four or five days. At first it is better to use excess of balsam, as this will be squeezed out and can be easily cut off when dry ; but with experience the quantity of balsam may be reduced.
The filters for three-colour work are innu merable, the following being satisfactory : Hoechst Dyeworks' subtractive filters for three-colour printing of all kinds.
Blue-violet Stock Solutibn Crystal violet . . 61.7 grs. 4 g.
Warm distilled water I 2 OZ. mins. 35o ccs. Glacial acetic acid . 5-6 mins. 5-6 drops Filter Dye solution 338 minx. 20 CCS.
Gelatine solution (6 %) 31 oz. roc) Or Stock Solution Rapid filter blue . 151 grs. i g.
Hot distilled water 6 oz. 16o mins. 18o ccs. Liquor ammonia . 8 mins. 8-io drops Filter Dye solution . 338 mins. 20 CCS.
Gelatine solution . 3- oz. zoo This is faster to light than crystal violet.
Green Stock Solution Rapid filter green I . 62 grs. 4 g.
Distilled water . 3 oz. roc) ccs. Filter Dye solution . . 338 mins. 20 CCS.
Gelatine solution (6 %) 31 oz. roc This transmits a narrow band in the extreme red ; the following does not, and should always be used with panchromatic plates : Stock Solution Filter blue green . 151- grs. I g.
Rapid filter yellow . 151 I Distilled water . 31 oz. roc) cu. Filter Dye solution . . 338 rains. 20 CCS.
Gelatine solution (6 %) 31 oz. roo Red Stock Solution Rapid filter red I . 77 grs. 5 g.
Distilled water . 7 oz. 200 ccs. Filter Dye solution . . 338 minx. 20 CCS.
Gelatine solution (6 %) 31 oz. roo Allow 118 minims to every 16 sq. in., or 7 ccs. to every roc qcm., and cement two glasses of each colour together. The ratio of exposures for pinachrome or pinacyanol bathed plates is, for the blue screen (yellow printing) negative, 4 ; for the green screen (red printing) negative, 8-12 ; for the red (blue printing) negative, 8-12.
For liquid filters the following, suggested by Newton and Bull, for use with panchromatic plates, with cells of I cm. internal thickness : Blue Filter Victoria Blue B (Bayer) (I% sol.) 448 rains. 47 ccs.
Naphthol green (1% sol.) 174 8 Distilled water to . 20 oz. 1,000 This will not keep well when mixed.
Green Filter Rapid filter green (r% sol.) . . . 87 mins. 9 ccs.
Naphthol green (r% sol.) 87 9 Rapid filter yellow K (1% sol.) . . 87 9 ,, Distilled water to . 20 OZ. 1,000 Red Filter Rose Bengal (1% sol.) 442 mins. 84 ccs.
Rapid filter yellow K (1% sol.) . . 442 84 Distilled water to . 20 oz. r,000 The above are to be used for the subtractive process or three-colour printing, whether on paper or in the shape of superimposed stained transparencies for projection. For the pro jection of transparencies by means of coloured lights or the additive process, the following should be used for obtaining the negatives : Violet-blue Stock Solution Crystal violet . . 62 grs. 3 g.
Methylene blue I IP Distilled water 8 oz. 384 mins. 25o ccs. Glacial acetic acid . 5-6 mins. 5-6 drops Filter Dye solution . . 338 mins. 20 ccs.
Gelatine solution (6 %) 3i oz. 100 Green Stock Solution Rapid filter green 2 . 62 grs. 4 g.
Distilled water 4 oz. zo7 mins. 120 ccs.
Filter Dye solution . . 338 mins. 20 ccs.
Gelatine solution . 3i oz. 100 This transmits a narrow band in the red, but the following does not : Stock Solution Filter blue green . 18+ grs. 1.2 g.
Rapid filter yellow . 2.5 Distilled water . 7 OZ. 200 ccs.
Filter Dye solution . . 338 mius. 20 CCS.
Gelatine solution . 34. oz. 100 Stock Red Solution Rapid filter red 2 . 77 grs. 5 g.
Distilled water 7 oz. 200 ccs.
Filter Dye solution . . 338 mins. 20 ccs.
Gelatine solution (6 %) 3+ oz. 100 The quantity of dyed gelatine per area is the same as for the subtractive filters, and two filters of like colour must be cemented together. The ratio of exposures with pinachrome or pinacyanol bathed plates is blue 4, green 12, red 12.