44 



On the Reflection of Light in the Atmosphere. 



These values we will here apply. 



We will uow compare the most complete of the above three 

 series of observations, those made at Heidelberg, with the theo- 

 retical formulae. For this purpose the coefficient a must be 



S 

 determined from one of the observed values of ^r • We will 



rl 



take the value which corresponds to the solar height 40°, 



because 40° is just the mean between 20° and 60°. This value 



is 1, and from this we get 



= 0-586. 



Introducing this value for a into the equation (3), we can cal- 



S 

 culate yy for other solar heights, and thus obtain the following 

 H 



values, which may be again collated with the observed values : — 



Sun's altitude. 



Calculated. 



Experiments. 



o 



20 



38 



035 



25 



0-55 



0-48 



30 



071 



0-t!5 



35 



0-86 



0-82 



40 



1 



1 



50 



1-24 



1-37 



60 



1-43 



1-60 



If it be considered that in this calculation several cooperating 

 circumstances have been disregarded, and that no perfect accu- 

 racy is to be ascribed to the observations, the agreement between 

 the calculated and the observed values will be found to be suffi- 

 ciently close. There is therefore no reason for saying that by 

 these observations the hypothesis that the reflection is effected by 

 the water-vesicles in the atmosphere is refuted. 



I think, moreover, that comparisons of this kind are little 

 fitted to decide on the accuracy or inaccuracy of the hypothesis ; 

 for the other cooperating circumstances, which can be but im- 

 perfectly taken into account in the calculation, have partly a far 

 greater influence on the intensities of light than the nature of 

 the reflecting constituents. 



I remain, Gentlemen, 



With great respect, 



Yours, 



R. Clausius. 

 Zurich, May 26, 1866. 



