p. LECOMTE DU NOiJY 747 



lamp was very small, as compared to that radiated beyond. From 

 7 /i to 10 n, it amounts to less than 1 per cent of the total. As the 

 other errors involved by the method are of a greater order of mag- 

 nitude, it was adopted as the upper limit, for the total radiation. 



Limit between the Red and the Infra-Red. — (Upper limit of inte- 

 gration of the visible spectrum). Langley, although he does not 

 specify it, seems to have chosen 0.75 ix as the upper limit. Many 

 workers have chosen 0.8 fx (as the eye is sensitive to the radiations 

 up to 0.8 ix). Some have preferred 0.76 ix, others 0.7 /x. The reason 

 for the importance of this determination is that energy increases 

 very much between 0.7 ix and 0.8 /x, whereas the impression on the 

 eye is very slightly changed. In other words, the shifting of the 

 limit from 0.8 /x to 0.7 /x will change considerably the amount of energy 

 spent in the visible spectrum, whereas the effect on the eye will 

 hardly be noticeable, since it only brings in very faint, deep red rays 

 which, if absent, do not modify one's impression appreciably. On 

 the other hand, if it is sought to determine the minunum of energy 

 necessary to make the red rays between 0.7 ix and 0.8 /x impress the 

 retina, one has to shift the limit as high as 0.8 ix. And in this case, 

 all the values given for the energy of radiations below 0.7 ix will be 

 altered (by more than 27 per cent). Therefore, in this paper, the 

 two figures are given, so that one may compare the results. 



The study of luminosity curves shows that, by removing the part 

 of the spectrum extending beyond 0.7 /x, the total luminosity is only 

 decreased by 0.4 per cent. As Konig and Brodhun (11) have shown 

 that the human eye was just able to detect a change in luminos- 

 ity when it amounted to 1.6 per cent, we feel that this limit is 

 adyisable. , 



Measurement of Total Radiation. 



The first step was to measure the vialue in absolute units of the 

 total radiation of the Nemst lamp, with which it was intended to 

 experiment; for it was difficult, owing to the discrepancies found in 

 the figures given by different authors, Lux (12), Hartman (13), 

 Ingersoll (14), etc., to rely upon data found in literature. 



The source was an ordinary Nernst lamp, (110 volts, 1.3 amperes). 

 In order to prevent any fluctuations due to cooling by air currents, 

 the glower was enclosed in a brass chamber, with just one rectangular 



