OP ARTS AND SCIENCES. 



19 



The first column gives in each case the meridian distance, the second 

 the mean of the four j^ositions of equality of the Nicol's prism, and the 

 third the corresponding polarization. All these observations point to 

 one very remarkable result ; namely, that the polarization is the same 

 for a given solar distance for any meridian distance ; in other words, 

 that the polarization is the same for all points equally distant from the 

 The variations in the observations are to be ascribed partly to 



sun 



errors of observation and partly to real irregularities in the atmosphere, 

 as it is evident that they follow no regular law. The means therefore 

 give us the true polarization with much greater accuracy. They are 

 represented in Fig. 5 by small crosses. The next thing is to deter- 

 mine the law which connects the polarization with the solar distance 

 in all these observations. A drawing was made like Fig. 5 enlarged, 

 and a fine copper wire laid on it, and bent into such a shape that it 

 should coincide as nearly as possible with all the observations. The 

 ordinates for every 10° were then read off, giving the results entered 

 in column 2 of Table XII. 



TABLE XII. 



Theoretical Formula for Sky Polarization. 



A simple exjDlanation of the polarization of the sky is to assume that 

 it consists of molecules of air or aqueous vapor, which reflect the light 

 specularly, and whose index of refraction differs only by a very minute 

 amount from that of the medium in which they float. The theoretical 



