790 WALKER. 



300 /JL^JL in wave length. He examined the spectra of various high 

 power are lights with opal and milk glass shades or coverings. These 

 spectra compared very closely with that of cloud light. 



Hertel and Henker ^^^ in support of this view carried out a very 

 accurate set of measurements, in the laboratory of C. Zeiss, Jena, of 

 the cloud and skylight spectrum, of variously covered or shaded high 

 power arc lights and of the percentage of penetrability and absorp- 

 tion power of various glasses for wave lengths in different parts of the 

 spectrum. The spectrum of the clouds or clear sky was found to 

 contain no rays below 300 nfx and very few below 310 ju/x. 



Having what they considered an ideal light the next question was 

 in what manner should the artificial lights be compared with it. In 

 producing injurious effects on animals they noted that unprotected 

 lights of very great intensity had been used under conditions never 

 found in modern lighting systems therefore previous observers had 

 studied entirely atypical conditions and not the conditions to which 

 human eyes are really exposed. Widmark had used a 1200-4000 c. p. 

 arc light without covering at a distance of 25 cm. from the animal's 

 eye for length of time ranging from 2-4 hours and longer. Likewise 

 Birch-Hirschfeld had used similar arc lights with and without disper- 

 sion through a prism usually at a distance of 10 cm. to 20 cm. Hess 

 had used a Schott uviol lamp 65 cm. long of 3-3| amp. The animals 

 were at a distance of 10 cm. to 20 cm. and the time of exposure 1-16 

 hours. These data represented the average experimental conditions 

 necessary to produce injuries but not at all the conditions to which 

 mankind is at present exposed. 



Therefore they considered a far better criterion for practical pur- 

 poses would be accurately to photograph the spectrum of arc lights at 

 the minimum distance of actual service in lighting, and to find whether 

 or not such globes or mantles can be placed around the light source as 

 to render the spectrum, in cjuantity and quality, within the range of 

 the cloud or skylight spectrum. 



Accordingly the lights were placed 50 cm. to 100 cm. distant from 

 the spectroscope opening. The optical system of the spectroscope 

 was made of quartz glass. The effect of indirect illumination and of 

 half spherical or inverted bowl shaped shades for increasing horizontal 

 illumination, was also measured by raising the light and its shade 

 about 40 cm. above the axis of the spectroscope condensor. 



In all cases under these conditions by use of the usual milk glass 

 and opal glass shades they had no difficulty in getting a spectrum 

 comparing in quantity and quality very closely with the spectrum of 



