200 Mr. G. Zettwuch on the 



It is thus easy to understand how under such circumstances 

 no general law is capable of representing the phenomenon. 



" I wish further to draw attention to the fact that even with 

 an apparently serene and immutable sky different values have 

 been obtained for n in the same set of observations, and the 

 differences appear to exceed the limits of errors of observation. 



" 2. Considering merely the results obtained on clear days, 

 such as those on July 11, 16, and 17, and on October 6, 

 it is seen that the value of n increases continuously with 

 the zenithal distance of the sun ; its minimum appears to 

 coincide with the minimum daily zenithal distance. The 

 variations are best seen by constructing a curve whose 

 abscissae represent zenithal distances and ordinates the calcu- 

 lated values of n. [Two such curves, corresponding to 

 July 17 and October 6, are given.] 



" 3. These curves show that for large zenithal distances, 

 between 70° and 90°, n increases fairly rapidly. 



" 4. The composition of the zenithal light cannot be repre- 

 sented by a single general formula. In rare instances the 

 results approximate to Eayleigh's law ; however, it is clearly 

 seen that, for zenithal distances in the neighbourhood of 90°, 

 they show no great divergence from it, and tend towards it 

 rapidly. It is further also seen at a glance that it is not at 

 the zenith that the blue has the maximum degree of satu- 

 ration ; but that this always occurs at an angular distance of 

 90° from the sun, a result which also follows from Wild's 

 observations. 



" I have as yet been unable to take observations with a 

 very deep-blue sky ; even on the days when the highest values 

 of n were obtained, the sky did not appear to me to be of an 

 intense and deep blue. 



"According to my results it appears that the blue of the 

 sky at Rome is less pronounced than in England, Germany, 

 and France. 



" This conclusion may, however, be somewhat premature. 

 It is necessary to wait for the collection of a larger number 

 of observations, made at a more favourable time than was 

 possible to myself — since the past year has been an excep- 

 tionally rainy one. Moreover, it becomes necessary to consider 

 the small altitude of the observation-station above sea-level 

 (60 m.), and its surroundings. 



" 5. The perturbing cause which in most cases produces 

 variations in the composition of the diffused light is un- 

 doubtedly to be found in the clouds ; which, though small in 

 numbers and very thin, disturbed the relative intensity of the 

 two spectra in a surprising manner as they approached the 



