On the Relative Luminous Intensities of Sun and Sky. 555 



in the flash which are present in the emission-spectra, inter- 

 esting comparisons can be made with photographs of the 

 actual flash-spectrum of the sun. I am also engaged in 

 making accurate determinations of the dispersion of metallic 

 vapours by means of a metal prism of 45 degrees, furnished 

 with mica windows. The prism is filled with hydrogen, and 

 the metal — say sodium — vaporized in this atmosphere by 

 the application of heat. The results obtained in this way are 

 far superior to those yielded by prismatic flames. The angle 

 of the prism is accurately known, and it is filled with non- 

 luminous sodium vapour of uniform density and under known 

 conditions of temperature and pressure. With this I have 

 obtained much greater curvature of the spectrum in the 

 vicinity of the absorption-lines than that figured by Julius 

 (the total lateral bending in one case being 400 times the 

 distance between the D lines), and the spectrum is perfectly 

 steady instead of fluttering, as is the case when the deviation 

 is effected by means of a sodium-flame of prismatic form. 

 The work along these lines will be reported in a subsequent 

 paper. 



University of Wisconsin. 



LIII. On the Relative Luminous Intensities of Sun and Sky. 

 By Quirino Majobana*. 



THE problem regarding the cause and intensity of the light 

 coming from the sky has been dealt with by numerous 

 writers. Newton was the first to point out that under certain 

 conditions some bodies appeared to be coloured, without, 

 however, possessing any coloration properly so-called. 



In the case of the atmosphere, the most probable hypothesis 

 would seem to be that according to which the light coining 

 from the sky is due to the reflexion of sunlight from particles 

 suspended in the atmosphere. Strong support is given to this 

 theory by the fact that the light coming from the sky is 

 polarized. This, however, does not, according to some, con- 

 stitute an argument in favour of the view that the coloration 

 of the sky is not a true coloration of the atmosphere. 



Whatever the explanation may be, it is in any case interest- 

 ing to ascertain the value of the luminous intensity of the sky. 

 This intensity might be expressed in terms of any arbitrary 

 unit ; but it is much simpler to refer it to the luminous 

 intensity of the sun, no matter in what units the actual 



* Translated from the Rendiconti de.lla R. Accademia deiLincei, Classe 

 di Scienze lis., mat. e nat., vol. ix. 2d >->em., Ser. I , fase. 3, from a sepa- 

 rate impression communicated bv the Author. 



