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RADIATION CHARACTERISTICS OF TWO MERCURY ARCS
FRANK BENFORD* INTRODUCTION
The lighting industry from its earliest beginnings to the present time has been under a peculiar handicap because man's best efforts in the production of light have always yielded results different from the light of nature under which mankind has developed. It is no mere accident that the human eye is most sensitive to those rays of the sun that are received most abundantly at the earth's surface. This condition may be looked upon as a natural result of evolution and, if the facts were otherwise, it would be a matter for profound speculation. But while the eye has adapted itself to the light of the sun, there are other natural and artificial factors to be considered.
The beneficial effects of natural light, and of sunlight in particular, have long been known, but it is only recently that definite data have been obtained on the so-called therapeutic rays, and these rays are found to lie entirely outside of the visible spectrum. The rays that tan the skin and promote the growth of the bones lie at the extreme boundary of sunlight as received at the surface of the earth. In this case, our logic of connecting each function of the body to such a part of the spectrum where it will lie under optimum conditions is rather weak, and perhaps in abler hands it might be shown that the presumably older function of the bone growth having a maximum sensitivity to radiation at 2967 A., as contrasted with the newer sensation of vision with its peak at 5550 A., indicates that in previous geologic ages the composition of sunlight was decidedly stronger in the far ultra-violet regions.
A third factor, and the one in which this Society is particularly interested, is the region between the therapeutic range and the visible range, that is, from 3200 A. to 4000 A. Here the ordinary photographic emulsion is most affected, and therefore to people who are interested in health, photography, and vision there may be
* Research Laboratory, General Electric Co., Schenectady, N. Y. 404