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Fig. 1

For comfortable working in the dark room one needs as much light as is consistent with safety, which means that the color and the amount of the illumination that can be used depend on two things - the sensitiveness of the plate and the sensitiveness of the eye.

Plates vary according to their kind, while of the 4 per cent, or less of people who do not possess normal vision, the majority are slightly deficient in their perception of red, a few have the opposite defect and to them red is irritating, while a few others are deficient in the perception of green or blue.

By passing a ray of white light through the spectroscope, we obtain a band of pure colors in the following order: Violet, blue, green, yellow, orange and red. This fact must be introduced to make any discussion of dark room illumination possible in even the most elementary scientific manner.

As we cannot show the spectrum in colors, we have drawn a diagram showing roughly the position of the colors as they appear in the spectrum (Fig. 1).

The second diagram shows how the colors of the spectrum affect the eye when very bright and very weak lights are used. If you were to look at a brilliantly illuminated spectrum, the yellow or yellow-green would seem brightest to the eye, as shown by the upper curve in our diagram. It is also seen that the line falls away almost equally in the red and green, showing that these colors are equally bright to the eye while its lowest point is in the blue or the violet, showing that these colors are darkest to the eye.

Reduce the quantity of light and the relative brightness of the colors change as shown in the lower curve, green appearing brightest, while red, orange,yellow, blue and violet are all darker. This phenomenon is called the Purkinje effect, from the professor of that name who discovered it, and it must be considered if eye strain is to be taken into account in using a weak dark room illuminant.

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Fig. 2

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By Edward H. Weston Tropico, Cal.

On the other hand, all plates have the greatest sensitiveness to the violet-blue, though the orthocromatic or isochromatics have an additional sensitiveness to the green and yellow and the panchromatic is sensitive to all colors. The diagrams (Fig. 3), show the sensitiveness of these plates to the spectrum, the height of the curves showing the proportionate effect of various colors. Actual photographs of the spectrum made on the three plates are shown in Fig. 4.

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Fig. 3

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Fig. 4

Although the ordinary plate is most sensitive to blue, with sufficient exposure it is also sensitive to green, and with a very long exposure even to red, so it will be seen that no light can be made absolutely safe to work by for an indefinite time.

Because a plate is very sensitive, and the eye very insensitive to blue, this color cannot be used for a dark room light, as the smallest amount will cause fog. A green or red light may be used, or a mixture of green and red, that is, a yellow, though it would have to be a very deep yellow. But it must be clearly understood that there is no illumination at all that is absolutely safe for an indefinite time.

Until 1907 when Dr. C. E. K. Mees made an investigation of the subject, there were no dark room light standards. The photographer bought ruby or orange fabric or glass and was satisfied so long as he got no fog in his plates. He did not know whether he was getting as much light as it was possible to use with safety or not. In fact, dark room lights were much darker than they need have been. The photographer did not realize that he might have had much more light and worked in comfort with safety, but if he had known it such safelights were not then to be procured.The standard of safety that Dr. Mees adopted allowed for a plate being exposed half a minute at three feet from the dark room lamp without showing any trace of fog, the lamp used being eight candle power when placed directly behind the safe-light or sixteen candle power if the light did not come direct but was reflected through the safe-light.

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By Edward H. Weston Tropico, Cal.

More light could be used if the plate was further away or was not exposed so long, while on the other hand if a less powerful light was used the plate could be exposed a longer time or nearer the lamp.

This standard was fixed for a plate in a dry condition. Of course development takes more than half a minute, but it is never necessary to expose the dry plate to so strong a light longer than this. Plates are usually removed from the holders by a dim light and development is not carried on in the direct rays of the light. As a plate is not so sensitive when wet the margin of safety allowed in the standard is materially increased.