NON - ACTINIC flEDIA.—Media for the transmission of non-actinic rays of light. The color of this depends greatly upon the nature and sensibility of the material to be handled.
It vanes from a deep ruby to a pale orange. Abney recommends bookbinders' red cloth combined with stained red glass. It should be noted that ruby glass by itself will rarely be found sufficient, since a certain amount of blue light is apt to permeate it.
Many advocates have been found for a paper treated with lead chromate, and termed can ary medium. For developing or dinary plates two thicknesses of this substance will be found sufficient. There is, however, an orange paper which will be found useful, as it retards more of the green rays.
It is well to note that if a plate be first exposed to white light and afterwards to red, a reversing action will occasionally take place, the image eventually disappearing. It is, therefore, necessary to avoid handling the exposed plate in the red light as much as possible until com mencement of development.
The value of the light coming through stained, red, and ruby glass, orange paper, the canary medium will be seen on reference to Fig. 298.
Spectroscopic tests r e c e n tly made have shown that few of the commercial so-called " non actinic " glasses are as safe as could be desired. Vogel sug- • gests that ruby glass so difficult to obtain should be replaced by colored gelatine films. His method of manufacturing these is as follows :-1 gramme of aurantia is dissolved by aid of heat in ioo c.c. of distilled water. Unless the whole of the aurantia dis
solves, a few drops of ammonia should be added. 20 grammes of gelatine are also dissolved in loo c.c. of water. 25 c.c. of the aurantia solution are taken and mixed with 25 c.c. of the gela tine solution, and the mixture filtered through flannel. With this solution leveled glass plates are coated. These plates absorb the blue rays only, and are therefore only suitable for use in developing wet collodion or low sensitive gelatine emulsion plates or paper, as they allow the yellow, green, and red rays to pass through freely. For development of highly sensitive plates rhodamine should be used, as a concentrated solution of it absorbs yellow, green and violet completely, whilst blue and red light pass through freely. 8 grammes of rhodamine are dis solved in 250 c c. of water, and zo grammes of gelatine in too c.c. of water. 25 c.c. of the gelatine solution are then mixed with 3o c.c. of the rhodamine solution, and the mixture filtered. It we combine a glass plate coated with this solution with one made with aurantia as previously described, we get a perfect nonactinic medium, permitting only red light to pass through. The two plates when dry are fixed together, film to film. If, previous to coating the glass plates, they had been rubbed over with French chalk and then provided with a substratum of collodion, the films could be stripped from the glass when dry, and used for dark room lamps, etc.