BELL. — ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 



injuries ascribed to ultra violet rays, like cataract and retinal degen- 

 eration, are charged to the radiations running even up to the visible 

 spectrum, while the extreme ultra violet, absorbed by the cornea, 

 produces only superficial lesions generally recovered in a few days. 



From the standpoint of the present investigation it did not seem 

 justifiable to attempt to pass without further investigation on the 

 validity of any of the divergent views here noted, but to deal with 

 the radiations of short wave length as a whole, including in the possibly 

 injurious group all those radiations which have been under serious 

 suspicion on clinical evidence by reputable investigators. The line 

 has therefore been drawn between the ordinary lighting radiations 

 and radiations of short wave length in the extreme violet and ultra 

 violet of the spectrum, -where the lighting value of the rays is negli- 

 gible and their actinic value notably high. 



Separation of the Ultra Violet from the Visible Spectrum. — Having 

 determined on such a separation of the radiations under grave sus- 

 picion of injurious action from the rest of the spectrum, it was next 

 in order to find a suitable screen for making just this division of the 

 spectrum, so that it would be possible to measure the energy in the 

 two portions of the spectrum directly and as a whole, without a 

 resort to the extremely difficult and troublesome measures of the 

 energy in separate spectrum lines, a task of great delicacy when 

 discontinuous have to be compared with continuous spectra. After 

 considerable investigation a suitable medium was found in the so- 

 called Euphos glass. This glass, which has been strongly recom- 

 mended by Schanz and Stockhausen as eliminating completely all the 

 harmful Tays and which was prepared under the direction of one of 

 them, cuts off the ultra violet spectrum with remarkable definiteness 

 while showing relatively little absorption of the general luminous 

 rays. 



Plate 1, b, c, d, shows the nature of this absorption very clearly. 

 Spectrogram b of this Plate is the spectrum of the mercury c^uartz 

 arc put on merely for reference, the group at 365 jjLfx being at the right 

 of the figure and the brilliant green line exactly in the centre of the 

 plate. Spectrogram c shows the spectrum of the magnetite arc which 

 is very rich in the ultra violet and d shows the same as absorbed by a 

 Euphos glass screen 2 mm. thick. The exposure in each case was one 

 minute with a rather wide slit and a very brilliant grating. The cut 

 off of the shorter wave lengths by the Euphos glass in the ultra violet 

 is very clean and sudden at wave length 390 /jl/x, practically just at 

 the end of the visible spectrum as seen by the average eye. The 



