Comparison of Sources of Artificial Illumination. 109 



the block of magnesium carbonate were admitted through a 

 four inch aperture in the opaque shutter which otherwise com- 

 pletely closed the only window of the spectrometer room. 

 This window had a southerly exposure and the light reaching 

 the magnesium-carbonate was received almost exclusively from 

 the sky. Of the considerable number of comparisons which 

 we made on different days, only two are presented in Table IY. 

 One of these was made between two and three P. M. on a 

 cloudless day of the summer of 1888, the other on the day fol- 

 lowing, when the sky was completely overcast with very heavy, 

 dark, low-hanging clouds. Our other measurements all gave 

 values lying between these extremes and, since they show no 

 peculiarities which are not better exhibited by the latter, they 

 have been omitted from the table. 



Comparisons of daylight with artificial sources of illumina- 

 tion have likewise been made by O. E. Meyer * and H. C. Yo- 

 gel, f and similar comparisons of the direct rays of the sun by 

 Crova,f Meyer, Pickering, § Yogel, G. Muller, || F. Exner "f 

 and others. 



The results given in Table IY, where our own measurements 

 are placed side by side with those of Yogel, reduced to the 

 same scale, exhibit in the most striking manner the enormous 

 richness of daylight, as compared with any form of artificial 

 light, in the shorter wave-lengths. In the extreme violet, 

 (A=4440 and 4260), according to the measurements of Yogel, 

 day light by unclouded sky is relatively fifty and one hundred 

 times brighter than a petroleum flame. Scarcely less surpris- 

 ing is the difference in quality between the daylight of clear 

 and cloudy weather. The absorption due to moisture in the 

 latter case begins to be important in the neighborhood of the 

 F line, from which point it increases rapidly in amount to the 

 end of the visible spectrum. Our curve of intensities for 

 clear weather (see figure 3, curve YI) shows traces of this 

 same absorption in the violet. 



All the European observers, with the exception of O. E. 

 Meyer, whose measurements of the electric arc and of day 

 light and sunlight are widely at variance with those to which 

 reference has been made, agree in finding the absorption of the 

 shorter wave-lengths even more marked in the direct rays of 

 the sun than in daylight by clouded sky. Thus Yogel, whose 

 measurements may be taken as typical, gives for sun light the 

 following values, which have been reduced to the same scale as 

 those presented in Table IY. 



* O. E. Meyer, Carl's Zeitschrift fur Angewandte Elektrioitats-lehre, I, p. 

 -320, 1879. f H. C. Vogel, 1. c. % A - CrOTa , ] « c. 



£ W. H. Pickering, Photometric Researches, etc., p. 246. 

 | G-. Muller, Astronomische Nachrichten, ciii, p. 241, 1882. 

 TfP. Exner, Repertorium der Physik, xxii, p. 605, 1886, 



Am. Jour. Sci.— Third Series, Vol. XXXYIII, No. 224. — august, 1889. 

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