638 VERHOEFF AND BELL. 



in the visible and ultra violet portions of the spectrum, 35% being in 

 the visible spectrum itself and 65% in the ultra violet between wave 

 lengths 400 fxij, and 200 /x/j.. This 65%, is eciually divided between 

 wave lengths 400 ju/i to 300 /jlh and 300 /xju to 200 fxfx, as one of us 

 has already shown (loc. cit.). As the lamp was run the total radia- 

 tion of energy having wave lengths less than 400 (jlijl at a distance of 

 50 cm. from the tube was to a very close approximation 11,000 ergs 

 per second per square cm., of which 5,500 ergs per square cm. of 

 energy were of wave length less than 300 /x/z. At distances other than 

 the standard one here noted, the radiation follows the law of inverse 

 squares with substantial precision. A small correction should theo- 

 retically be applied because the radiating body is approximately a 

 cylinder instead of a point. But for all practical purposes this 

 correction may be neglected, since for a radiative body of the dimen- 

 sions used it amounts to less than one quarter of 1% at all distances 

 greater than 50 cm., and does not exceed 2% even when the distance is 

 reduced to 20 cm. Plate 5, Fig. 1, shows the actual spectrum of the 

 quartz lamp taken with a rather wide slit and prolonged exposure with 

 wave length scale annexed, and Figure 2 shows the stronger lines of 

 the visible spectrum alone. The lines toward the right of A, which 

 do not appear in B, are merely the ultra violet lines of the over- 

 lapping second order spectrum, the photograph having been taken 

 with a concave grating of 1 meter focal length and 10,000 lines to the 

 inch. Attention should be called to one interesting feature of this 

 mercury arc spectrum. It will be seen that there is but a single ultra 

 violet line between the strong double line at 313 /x/z and the strong 

 group at 365 nix and this line is relatively weak. There is also a 

 conspicuous gap between 313 and the next line at approximately 

 303 jjLiJi. These gaps in the spectrum are of some significance in inter- 

 preting bactericidal experiments in this region of the spectrum. 



Determination of Liminal Exposure. 



As a starting point in our experiments it was necessary to determine,, 

 using the standard source just described, how long exposure at some 

 known distance was necessary in order to produce clearly marked 

 symptoms of photophthalmia. Our experimentation throughout the 

 work has been chiefly with rabbits, since these animals have been 

 generally used by other experimenters and the characteristics of their 

 eyes have therefore become fairly well known. 



