Luminous Intensities of Sun and Sky. 



561 



to be of the same tint. But the matter is somewhat more 

 uncertain when the comparison is to be made between the 

 light of the sun and that of the sky ; especially if the difference 

 of colour is strongly marked, as at the crater of Etna, some 

 3000 metres above sea-level. In such a case it is necessary 

 to repeat the measurement a large number of times, re-setting 

 the diaphragm each time. 



With this apparatus, measurements were carried out at 

 Rome in the summer of last year ; they were repeated, with 

 greater frequency and regularity, at Catania during the same 

 period. The following are some of the results obtained. 



The luminosity of the sky varies from point to point- 

 Thus on a day in August, and about 11 o'clock in the morning, 

 the following values of r were found : — 



Ln the immediate neighbourhood of the sun . r= 357,000 

 About 90° from the sun, in arry direction . r= 950,000 



Almost on the horizon r= 98,000 



A white cloud r= 18,000 



These variations are evidently due for the most part to the 

 presence of aqueous vapour in the atmosphere. 



But the luminosity of the sky also varies throughout the 

 day. In order to ascertain this, observations extending over 

 a period of seven hours were made on the 5th of August, 1899. 

 The two tubes of the sky-photometer were placed at an angle 

 of 90° relatively to each other. The first tube having been 

 directed towards the sun, the second was moved towards the 

 least luminous portion of the sky. The following numbers 

 are the means of the observations made on that day : — 



Hours after Midnight. 



6 . . ?'= 175,000 



7 . . r = 250,000 



8 . . r = 489,000 



10 



r= 935,000 



12 (noon) . 

 15 . . . 



17 . . . 



18 . . . 



r= 950,000 

 r = 870,000 

 r = 563,000 

 7^225,000 



The table shows that the relative luminosity of the sky is 

 least during the hottest part of the day, and greatest at sun- 

 rise and sunset. 



But the figures given above change considerably with the 

 altitude. In order to study this point, observations were 

 made at the Etna Observatory*, from the 8th to the 13th 

 August, 1899. It was in the first place easy to ascertain the 



* The Etna Observatory is a little below the level of the crater. It is 

 2942 metres above sea-level, and its latitude is 37° 44' ]7". I am indebted 

 to Prof. A. Ricco for the hospitality offered to me, which enabled me 

 to make these observations. 



