64G VERHOEFF AND BELL. 



thelium was injured by exposures through the cornea, prove con- 

 clusively that 295 MM is not the limit for human cells, since the cornea 

 obstructs all waves less than 295 ^i^t in length. It has frequently 

 been assumed that there is no actual limit of abiotic action but that 

 the latter exists in a diminishing degree through the entire syectrum. 

 Theoretically this may be true, but practically it is not, as our experi- 

 ments show, and for the following two reasons, namely, first, that in 

 the case of the longer waves and moderate light intensities the abiotic 

 action is so slight as to be readily overcome by the physiological activi- 

 ties of the cells, and second, that in the case of the longer waves and 

 intensities theoretically sufficient to produce abiotic effects the cells 

 are destroyed by heat action, so that there is no opportunity for 

 abiotic effects to become manifest. The real problem may therefore 

 be stated to be the determination of the critical wave length for 

 abiotic action with light intensities just below those sufficient to pro- 

 duce injurious heat effects. 



For the experimental investigation of this problem the cornea and 

 lens are of all the tissues of the body the most suitable. This is so 

 because, owing to their great transparency, extreme light intensities 

 are required to produce heat effects in them sufficient to mask abiotic 

 effects. The conjunctiva and skin are far less suitable for this purpose 

 # because when the limit of abiotic action with respect to wave length 

 is approached the hyperemia due to heat action overshadows that 

 due to abiotic action. 



In this investigation it was necessary to abandon the use of the 

 quartz mercury lamp employed in the earlier experiments. As 

 already noted the spectrum of this source has conspicuous gaps in the 

 very region to be examined for the purpose in hand. It has no lines 

 of perceptible intensity between 334 ^i/x and the strong double line at 

 313 jLt^t. Then there is a further gap extending down to the group 

 having its center about 302.5 /x^ and another between this group and 

 297 iJ.fx. In fact there are only three rather widely separated lines 

 in this entire debatable region within which the limit sought was 

 known to lie. We therefore turned to the commercial magnetite 

 arc as the most convenient available source since this had already 

 been found by one of us to be particularly rich in the extreme ultra 

 violet. This lamp uses as active electrode an iron tube carrying a 

 compressed mixture of magnetite and of titanium oxide, in a pro- 

 portion of about 3 of the former to 1 of the latter, opposetl to a copper 

 positive electrode. The light is practically all derived from the arc 

 stream produced by the magnetite electrode. The spectrum of this 



