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XIX. Researches on the Blue Colour of the Sky. Extracts from 

 a Thesis for the Doctor of Science degree of the University 

 of Rome. By Giuseppe Zettwuch *. 



DR. ZETTWUCH'S paper, from which the extracts given 

 below are selected, is divided into three parts. In 

 Part T. the author gives a connected historical accouut of the 

 various theories which have been propounded to account for 

 the blue colour of the sky; Part II. deals with the present 

 state of the problem ; and Part III. contains an account of 

 the author's experimental researches on the subject. These 

 researches were directed mainly towards the verification of 

 Lord Rayleigh's theory, according to which the intensity of 

 the various radiations due to particles small in comparison 

 with wave-lengths which constitute part of the light of the sky 

 is inversely proportional to the fourth power of the wave-length. 

 The apparatus consisted of a Kriiss spectroscope fitted with a 

 Vierordt double-slit collimator, which enabled it to be used as 

 a spectrophotometer. 



"Results of the Observations. — The observations are referred 

 to the true mean time at which they were taken, and to the 

 distance of the sun from the zenith at the same time. More- 

 over, as the results calculated as explained in an earlier part 

 of the paper varied with the hour at which the observations 

 were taken and according to local conditions, it was thought 

 more instructive to calculate the value of n which would 



satisfy an assumed law -—instead of — - n and all the series of 

 A, A, 



observations are accompanied by a brief description of the 

 state of the sky." 



The author next gives a long table of results, and then 

 follow his 



"Conclusions. — 1. From the general nature of the obser- 

 vations and the table of results it is clearly evident that we 

 have to deal with a highly variable phenomenon, as the light- 

 reflected from the sky, and coining from the same point of it, 

 is of variable composition. All the observations, no matter 

 what the state of the sky, show that the predominant radia- 

 tions are those of short wave-length. But their proportion is 

 not even approximately constant. 



<; The divergence presented by the different series of obser- 

 vations is not surprising, as it occurs even in the same series 

 of observations when taken with a variable state of the sky. 



* Communicated bv Lord Kelvin. Translated from the Italian. 



