1145. Coating

Coating. Now we are ready to coat the glasses with the plain gelatin. This is best done by laying the glass down upon a leveled surface. The writer prefers to use drinking tumblers, turned upside down, for the purpose, as these are easily leveled up with the help of a small spirit level. The glass to be coated should be slightly heated, and then the hot gelatin poured in a pool in the center, quickly flowing out to the corners either by tilting from corner to corner, or by assisting with a strip of glass. The coated plate is then placed upon the leveled tumbler to cool and set, after which it may be put into a rack to dry.

1146. Dyeing

Dyeing. The following dye mixtures must now be prepared:

1147. Red Filter Dye Bath. -

Rose Bengal, Stock Solution..........................

1 part

Rapid Filter K, Stock Solution.......................

1 part

Distilled Water............................

6 part

1148. Green Filter Dye Bath. -

New Rapid Filter Green, Stock Solution ...........

2 parts

Naphthol Green, Stock Solution........................

2 parts

Rapid Filter K, Stock Solution...........................

1 part

Distilled Water............................

100 parts

1149. Violet Filter Dye Bath. -

Victoria Blue, Stock Solution.............................

1 part

Distilled Water............................

8 parts

To this must be added Rose Bengal, drop by drop, shaking up and examining between each drop or two until the blue color is changed to a slight violet.

1150. Now, into a suitable porcelain or glass dish, pour one of the dye baths and immerse into it one of the dried gelatin-coated glasses. In about five minutes, it should be taken out and washed for one minute, and a second plate immersed in the dye bath for a minute or two, and then washed.

1151. The color chart will now be required for visual testing of the absorption of our filter (it being assumed that the operator has correct color vision). Take the two dyed plates and, holding them slightly apart, bring them close to the eyes and look through them at the color chart, in a good light. This can only be done successfully by daylight. If it is the red filter we are making, on looking through it at the chart the blue and green patches should appear black; but, if such is not the case, then the palest-tinted plate should be replaced in the dye bath, and the testing repeated after a few minutes. If the orange and scarlet patches of the chart appear exactly the same depth of color, then the filter is too deeply dyed.

1152. When visually testing the green filter, the blue and purple patches should appear black, the scarlet nearly so, and the green patch should appear neutral gray. A slight trace of red passing through the green filter will not signify.

1153. When testing the violet filter, the yellow, orange, red, and green patches should appear black and the blue and purple, a grayish blue. The main purpose should be to keep the filters as pale as possible, in order that the exposures through them shall not be unduly long. After drying they should again be tested for absorption, and if satisfactory, they will be ready to seal together with Canada balsam. This is done by melting the balsam in a small lipped vessel, and pouring a quantity on to one of the gelatin surfaces of the filter, bringing the other plate quickly into position upon it, and squeezing the balsam out to the edges. After allowing to set for a day or two, they may be bound around the edges with any suitable binding paper or tape, and are then ready for use.

1154. In making these filters, one gelatin surface only may be dyed if desired, or fixed-out transparency plates may take the place of home-coated plates.