About 200 experiments were performed during six days on the following kinds of glass and combinations thereof: - Ruby red (flashed); orange; green, with faint tint of yellow; green, very deep and pure colour; green cathedral glass, with distinct tint of blue; green cathedral glass, deep green, with very faint tint of yellow; green cathedral glass, with decided tint of yellow; olive-green cathedral glass; brandy-coloured cathedral glass; lemon-yellow cathedral glass; Carbutt's ruby paper; aniline red paper; post-office yellow paper; acme red paint on glass; Winsor and Newton's rose madder on glass, etc. These experiments were performed in groups of seven, in such manner that while the time of exposure, intensity of light, sensitiveness of plate, and strength of developer, as well as the length of development, were exactly equal, the results served not only as standards for comparison, but as checks against possible errors. Exposures were made both to cloudy daylight and to artificial light during times ranging in the first case from half a second to five seconds, and in the second case, from 30 seconds up to five minutes.
The several kinds of glass and combinations were further tested by a very powerful spectroscope.
The subjoined values assigned to the several kinds of glass relate entirely to their ability to intercept those rays or the spectrum capable of affecting a pure bromide of silver gelatine plate, and have no reference to their capacity to transmit rays useful for vision in the dark room, a subject which was considered separately. Ruby red glass, single thickness, offers but little protection. A plate will be heavily fogged beneath it in half a second of day light, and in three seconds will be rendered completely black and opaque. The same results will be attained by artificial light (a students' lamp was employed) in from 30 seconds to 2 minutes.
Red glass, two thicknesses. A distinct impression of cloudy daylight will be made through this combination in 1 second, but at the end of 3 seconds will not develope up to more than one-third of complete capacity.
Double red with single orange proved to be slightly better than double red alone.
Red, and especially double red, with green, prevents the passage of both luminous and actinio rays.
Green glass is worse by far than red, and those varieties having a yellowish tint, and especially those having a bluish tint, are perfectly worthless.
Green with orange. If the green is deep and pure, is better than single red, and nearly equal to double thickness of red glass; but if the green glass is off colour by reason of either a bluish or yellowish tint, it is inferior to single red glass.
Double green with single orange proved to be rather better than double red glass, and this held true when one of the pieces was slightly tinted with yellow.
All the varieties of cathedral glass named proved to be absolutely worthless, a result which the spectroscope clearly foretold.
Carbutt's red paper is about equal to single red glass, but is inferior to the orange green combination. This paper doubled is about equal to double red glass, but is then nearly opaque to luminous rays also.
Aniline red, acme red, and rose madder (oil colour) are of no value whatever.
Having found the choice so far as opacity to actinic rays is concerned, to lie between the double red glass and the orange and green, it was ascertained that the latter combination with the green glass doubled was rather better than the double red. To ascertain the relative values of these combinations for practical illumination in the dark room, duplicate windows, glazed in the one case with double red glass, and in the other with orange and double green glass, were made for the dark room, which was then tested by both day and artificial light. Tha result was decidedly in favour of the double red glass. Printing which could be read easily at a distance of 2 ft. from the light through the double red glass could not be read at all at a distance of 6 in. from the same light through orange and double green glass.
The question then arose whether light sifted through two thicknesses of red glass was really any better than the same amount of light transmitted by a single red glass. To decide this point, light was allowed to pass through a single red glass during one minute upon a gelatine plate, and upon another part of the same plate covered by two thicknesses of the same glass the same light was allowed to act two minutes. The plate after development showed that two thicknesses of glass had rather more thoroughly obstructed the chemical rays than was effected by the single glass; yet the difference was too slight to be regarded as important. It therefore follows that a dim light, or short exposure to strong light, through single red glass is practically as safe as exposure to the same amount of light after transmission through two thicknesses of red glass.
While, therefore, red light appears to be safer than any other colour having the same value to vision, it is by no means destitute of power to initiate the decomposition of bromide of silver. Indeed, a positive was produced in 2 1/2 minutes by the light, of a student lamp, distant 10 in., through two thicknesses of the best ruby glass. On development, the plate was found to be rather overexposed. Of course the same result could have been obtained in about 1 minute through a single pane of red glass.
The conclusion is therefore forced upon us that up to the present time we have no really safe light for the dark room, and since every ray in the spectrum has been tried, the probability seems to be that we never shall have one. The only practicable method approximating safety is to expose plates in manufacture, handling, and development, as little to any kind of light as possible. Plates can be made, handled, and developed in almost perfect darkness. Of course, a glance at the plate in the various stages of these operations is necessary, but the long continued exposures to which plates are frequently subjected, under the impression that the light is "non-actinic" is a delusion to which is probably due most of the failures in dry-plate photography. The fault does not altogether lie with the operators. On the contrary, many of the plates in the market are hopelessly fogged in the factories, while few, if any, are really as free from this defect as it is possible to make them. (Dr. Garrison.)
For the covering of glass for dark rooms and developing lamps, Dr. Stolze proposes an emulsion made by dissolving 10 parts of nitrate of lead in 100 parts of water. To this solution is added, constantly stirring, either 6 parts of neutral chromate of potash or 4 parts of the bichromate. This emulsion is cooled, chilled, reduced into nodules, and washed. After being liquefied, glass plates can be coated with it in the same manner as ordinary emulsion plates. Such a light acts exceedingly well, and can be used with great safety for the handling and developing of plates. The colour is of a pure yellow, and appears very light to the eye. By giving a thinner or thicker coating, the colour and opacity can be easily regulated. To protect these plates from moisture, it is advisable to coat with a crude collodion to harden them, or a chrome alum bath will do as well; and to prevent cracking or tearing when exposed to the heat of a gas or lamp flame, a little grape sugar is added to the emulsion; this will make the coat-* ing pliable.
Another method for making red glass for the dark room was lately given by Cassau, thus: 5 grm. carmine are dissolved in 40 c.c. of ammonia solution; 2 grm. of picric acid are dissolved in 450 c.c. of water, to which 7 grm. of glycerine are added. In this last solution 50 grm. of hard gelatine are allowed to soak 1 hour, and afterward dissolved in the water bath. When the gelatine is thoroughly dissolved, the carmine is added to the mixture. While warm, it is applied to an ordinary sheet of glass or window pane with a wide brush. As soon as the first coat is thoroughly dry, a second or third can be applied until the desired density is arrived at.
Scola has fixed upon the following formula: 100 c.c. water, 5 grm, gelatine, and 1 grm, silver nitrate. Glass coated with this solution is exposed to light until it assumes a reddish brown tint. It is then washed to eliminate the silver nitrate. A surface is thus obtained through which the actinic rays do not pass. The coloration may be deepened by increasing the proportion of silver nitrate up to 3 or even 4 grm. Glass tinted in this way may also be used to shade the dark-room lantern.