Seed Plate Negative, Artura Print, by Bretzman, Indianapoliscial dark room glasses has given rise to the impression that all glasses were more or less defective in some property which might be termed "safety," and that it is possible to make a filter for a dark room lamp of indefinite safety combined with great intensity. If photographic materials were sensitive only to some particular colored light, as, for instance, blue and violet, and were entirely insensitive to all other colors, then this would be quite true, but, unfortunately, all photographic materials are sensitive to light of any color provided that the light is present in sufficient intensity, and a plate which normally is sensitive only to blue light is slightly sensitive to green light and to a lesser but still appreciable degree to red light, so that even a safelight which transmits nothing but the extreme red end of the spectrum will, if a sufficiently powerful lamp is used behind it, affect ordinary photographic materials.

Some years ago a number of experiments were made in which safelights were prepared which quite certainly passed no blue or violet light but only the regions of the spectrum which they were intended to pass. Then, by a special method the intensity of the light to the eye was measured and also the safety. The result of these experiments was to show quite conclusively that it is not possible to combine indefinite intensity with safety and that the conclusion to which all experienced photographers have come that it is best not to trust the dark room lamp but always to expose materials to it as little as possible, is based on the soundest practice and cannot be affected by any number of screens. There is, in fact, only one screen for a dark room lamp which can be described as indefinitely safe, and that consists of a sheet of tin which will not let any light through, so that one must decide what degree of safety shall be required, remembering that safety is only obtainable at the expense of light.

Different workers will undoubtedly differ as to how much safety they need according to their carefulness in working, the speed with which they can work, and according to how much examination they give the plates during development.

The fact, however, that no dark room lamp can be considered indefinitely safe does not affect the fact that one light will be better than another for a given material. The value of a particular safelight glass can be well measured by the intensity which it gives to the eye multiplied by the safety, that is, the length of time that the material can be exposed to it without fog. Viewed in this way, the most suitable safelight for gaslight papers will be a bright

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From Eastman Professional School Demonstration yellow, for bromide papers or positive film an orange, for slow plates an orange-red, for fast plates a deeper red, and for panchromatic plates a special shade of green, and all these should be made so that they do not transmit any blue, - a condition which is completely fulfilled by the Wratten series of safelights, which were made as a result of the experiments just described. A deep red glass, for instance, is twice as efficient for use with NC film as an orange red; that is, for equal safety you can have twice as much light, but if you use such a plate as the Seed 23, you will get greater efficiency by using a more orange safelight.

The standard of safety which we adopted was that three feet from the lamp no visible fog should be produced in one-half minute; that is, if the dry plate were half covered so that half was exposed to the light and half not and the plate was taken out of the box and exposed to the lamp at three feet distance, the brightness of the lamp should be increased until the first trace of fog appeared with 45 seconds exposure, the plate being quite clear with one-half minute.

Now, some people who are very careful in protecting their plates can use a brighter light than this and will no doubt hold that this standard of safety is unnecessarily great, while others who prefer to keep their plates in full light while working will say at once that a dark room lamp ought to be safe for at least five minutes with the plate two feet off, to which the only answer that can be given is: "Very well, in that case only one-tenth of the light can be used that our standard requires." Then some workers may prefer the light at 18 inches instead of 3 feet. In this case the 16 c. p. carbon or 25 watt Mazda lamp if changed to an 8 c. p. carbon or 10 watt Mazda, will be equally safe.

The safety of any dark room lamp runs parallel to its intensity, and when we have chosen a safe-light which gives the greatest efficiency for the material used, then all we can do is to cut down the intensity of the light until we have the safety which we require, and no further changing of the color is likely to produce any advantage over the color which has been chosen for the material by actual measurement.

C. E. K. Mees.

Research Laboratory, July 9, 1915.

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From Eastman Professional School Demonstration

That earliest slight break in home ties - the morning when the boy or the girl first trudges off to school! From that day, the changes are rapid. Every year you note them. And, almost before you realize it, there comes the severer sundering of those ties, when John or Mary with a cheery "Will be home for Christmas, sure," waves a stout farewell.Both of you are choking back sentiment. And afterward - how pictures, showing all the rapid transitions, do help.

There's a photographer in your town. Eastman Kodak Company, Rochester, N. Y. Run in the September issues of the Ladies' Home Journal, Harper's, Century, Scribner's and Fra. This copy is timely NOW for your advertising - with your signature substituted, of course.

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By R. F. Hildebrand Chicago, Ill.