742 VERHOEFF AND BELL. 



impurity lines within the very short wave lengths and owe their 

 considerable proportion of ultra violet to radiation just outside the 

 visible spectrum. From the standpoint of effects upon the eye the 

 ultra violet region may be divided into two sharply separated portions, 

 one of which produces abiotic effects while the other does not. We 

 have for the first time definitely established the line of partition 

 between these two portions at 305 /x^t. Some of the earlier experi- 

 menters in this field imagined that they had detected abiotic effects 

 with slightly longer wave lengths, an error apparently due to insuffi- 

 cient knowledge of the absorbing screens which they employed, which 

 with rare exceptions are described, if at all, in very loose terms. 



No injurious effects have been attached with any reasonable degree 

 of certainty to the ultra violet radiation which lies between the end 

 of the visible spectrum and the beginning of the abiotic rays. Since 

 this range of radiation is present in considerable amount in ordinary 

 sunlight, it is sufficiently obvious that any definitely harmful results 

 producible under ordinary conditions would have been eliminated by 

 the ordinary progress of evolution. Artificial illuminants under any 

 practical conditions of use expose the eye to much less severe radia- 

 tion in this part of the spectrum than does ordinary daylight and a 

 fortiori can be excluded as possible sources of harm. 



With respect to abiotic radiations we have every reason to acquit 

 on sound experimental basis every known artificial illuminant when 

 working under the ordinary conditions of commercial use. Even the 

 c^uartz mercury lamp, which is per se richer in abiotic radiations than 

 any other commercial source of illumination, when equipped with its 

 ordinary globe is not only less rich in ultra violet per candle power 

 given than any other source, but is as we have found by experiment 

 incapable of producing any abiotic effects on the eye even after six 

 hours exposure at 30 cm. from the tube. We have further shown 

 that even where the lamp is used without its globe, a condition which 

 is avoided in consideration of efficiency, long exposures would still be 

 necessary to produce any injurious effects at any distance reasonably 

 to be expected. The same immunity from danger attaches to all the 

 sources at present in commercial use. It is well within the bounds 

 to say that there is no commercial illuminant from which the least risk 

 of abiotic radiation is incurred under the circumstances of practical 

 use. The only sources used in the arts which have abiotic power 

 enough to require special caution in their use are those not employed 

 for the purpose of illumination. Such, for example, are the powerful 

 arcs used in some electric welding processes, those employed in the 



