5()2 • On the Relative Luminous Intensities of Sun and Sky. 



fact that — except for regions close to the horizon — the lumi- 

 nosity of the sky had a sensibly constant value in all directions 

 when viewed from the summit of Etna. 



Further, points in the neighbourhood of the sun gave the 

 same value of r as those in any other region*. 



It was also found at this altitude that the value of r in- 

 creases from morning to mid-day. Thus, on the 11th of 

 August, 1899, when the morning was an exceptionally clear 

 one, the following observations were obtained : — 



Time. r. 



5-50 2,570,000 



7 3,125,000 



8 3,650,000 



9 3,930,000 

 11 3,760,000 



Thus the value of r at the crater of Etna is about five times 

 greater than in Catania. 



Almost simultaneously with these observations, Sig. Gau- 

 denzio Sella carried out a similar series on Monte Rosa. He 

 used the photometer described above, consisting of plain card- 

 board tubes, thus realising the apparatus suggested by Lord 

 Kelvin. He found values of r oscillating about 5,000,000, 

 and thus not differing very greatly from those found for 

 Etna. 



Considering the probable connexion between the quantity 

 of aqueous vapour in the atmosphere, in the form of a light 

 mist, and the luminosity of the sky, an attempt was made to 

 ascertain whether the hygrometric state of the place of 

 observation had any influence on the clearness of the sky. 

 With this object in view, almost all the observations made 

 were accompanied by determinations of the absolute and 

 relative humidity, by means of a good wet-and-dry bulb 

 hygrometer. No relation, however, was found, probably 

 because the hygrometric state of the lower atmospheric strata 

 may be quite independent of that of the upper ones, to which 

 the colour of the sky is due. 



* It is, however, to be noted that it is impossible to push the observa- 

 tions to points in the immediate neighbourhood of the sun, since there is 

 then the danger that the light coming- from the sun ma)' enter the sky- 

 tube. 



