Blue. Colour of the Sky. 201 



zenith ; as they passed the zenith, they rendered any adjust- 

 ment impossible by reason of the variations in the intensity 

 which occurred while the observations were being taken. 

 When the movement of the clouds took place between the 

 horizon and the zenith, the values found for n from different 

 readings of the same series of observations were markedly 

 discordant, as is shown by the table. 



"6. A cnrions result is obtained by comparing the values 

 found for n with the relative humidity u. This comparison 

 shows that to a high value of u there also corresponds in 

 general a high value of n. This parallelism does not appear 

 to me to have anv clear sionificance. since n has a larger value 

 in the morning than evening, and the relative humidity in 

 the neighbourhood of the soil is then also greater than at any 

 other time in the course of a fine day. The conclusion, 

 therefore, cannot be regarded as reliable. 



"When ultimately analysed, the results obtained do not 

 admit of an absolutely definite conclusion that at no time does 

 the composition of the diffuse light of the sky coming from 

 the zenith satisfy Rayleigh's law. It must be stated that 

 under certain circumstances, in a perfectly transparent atmo- 

 sphere and at a suitable angular distance from the sun, the 

 value corresponding to Rayleigh's law is undoubtedly con- 

 tinuously approached, and in certain cases surpassed. 



'• But if the observations on the diffuse light of the skvhave 

 in the case of every observer given variable and complex 

 results, those on the blue light emitted laterally by a suitable 

 turbid medium, and in particular on the artificial sky-blue of 

 Bock, have yielded reliable and uniform results. This will 

 be clearly seen from the values which I have calculated for n 

 from the observations of Bock [referred to above] ; the calcu- 

 lation being conducted in a manner similar to that used in my 

 own experiments. 



Blue of tlie Sky. 



Blue 



of Vapour Jet, 



>* yellow = 2*45 





3*99 



n green = 4*08 





3'9l! 



^blue = 3*09 





3-99 



"violet = 4'26 





4-00 



u On the one hand, for the blue of the sky the values of n 

 corresponding to the same series of observations exhibit a 

 considerable amount of divergence ; on the other, the values 

 found for the vapour-jet are in perfect accord, and yield the 

 value for n required by Rayleigh's theory. 



" From the fact that, on account of the variations taking 



