730 VERHOEFF AND BELL. 



from the cut-off of secondary radiation of relatively very long wave 

 length, 4 to 5 n and more, received from radiating surfaces at and below 

 red heat. The addition of 5 cm. of distilled water in filling the cell 

 reduced the transmitted radiation to 33%, which represents the radia- 

 tion between the former limit and approximately 1 ^u. There is every 

 reason to believe that most of this energy is from the hot body radia- 

 tion rather than from the line spectrum of the arc itself, for in so far 

 as known metallic spectra are not rich in intense infra-red lines. 

 Screen No. 1 then, cuts off 22% of the energy between 1 /x and the 

 extreme ultra violet. Screen No. 2, medium optical flint, is relatively 

 transparent, almost as transparent as the light crown screen No. 7, 

 while the light flint screen No. 4 cuts oft' more energy than either of 

 these. Used without the water cell all the four screens mentioned 

 cut off between 50 and 60 % of the total energy due to the large 

 absorption of glass in the extreme infra red corresponding to the 

 secondary hot-body radiation of the source. 



This secondary radiation being from a very diffused source cannot 

 have a conspicuous effect in those experiments in which the light 

 was concentrated through lenses although it comes into play in the 

 free radiation from the arc. It is quite certain, for instance, that in 

 the bactericidal experiments with the mercury arc which one of us 

 has recorded, in which trouble was experienced from heating of the 

 water in which the bacteria were suspended, this heating was mainly 

 from the large secondary radiation which is readily stopped by water 

 rather than from the characteristic radiation of the mercury vapor. 



The data heretofore given by one of us on the proportion of ultra 

 violet energy in the quartz mercury lamp and the magnetite arc may 

 be regarded as substantially correct for the metallic spectra as such, 

 the quartz water cell of 1 cm. thickness employed in these experiments 

 cutting off the secondary radiations rather completely without inter- 

 fering materially with the energy of the line spectrum. In our 

 experiments involving total thermic effects necessary corrections for 

 the conditions of the experiment can be made by reference to the 

 foregoing table. 



As regards the thermic action of radiation on the eye, there is no 

 reason to suspect any specific effects with respect to wave length. 

 So far as the action is not definitely abiotic or concerned with the 

 stimulation of the light perceiving functions of the retina it seems 

 to be purely a question of energy as in any other case of heating. 

 The more violent phenomena produced by heating are considered in 

 our discussion of eclipse blindness and allied phenomena (page 720). 



