﻿M. H. Wild on the Absorption of Light by the Air, 297 



8th of March, 1866, a long projected experiment for measu- 

 ring the absorption of the air by means of my photometer, which 

 gave, however, a negative result. I shall subsequently revert to 

 it. I at first concluded from it that the transparency of the 

 air was. so great that the enfeeblement of the light in a layer a 

 metre in thickness was below the limit of the delicacy of my in- 

 strument — that is, did not amount to yooo °^ tne incident light. 

 I therefore determined to make measurements in the open air at 

 far greater distances. After some preliminary experiments which 

 indicated the precautions to be taken, the definitive observations 

 were made on the 6th to the 10th of July 1866, first in my garden, 

 and afterwards in an open street in the neighbourhood of my 

 house. Two paper screens, consisting of square wooden frames 

 respectively 0*6 metre and 1*2 metre in the side, over which was 

 stretched paper from the same roll, were first put up side by side at 

 a distance of 6 metres from the two apertures in my photometer* 

 by which light enters, and the ratio of their brightness was 

 determined. Without altering the position of the smaller one, I 

 first placed the larger screen at a distance of 21 metres and then 

 of 36 metres from the photometer, again determined the relative 

 brightness by its means, which operation was once more repeated 

 after the screens had been again placed at equal distances. The 

 mean of the first and last measurements (with an equal distance 

 of the screens), compared with the results of the second observation 

 (when the screens were respectively 15 and 30 metres apart), ren- 

 dered it possible to determine the coefficient of transparency a of 

 the air. For if we designate by e the constant distance of the 

 small screen from the photometer, and the varying one of the 

 larger screen by E, if, further, the brightness of the small screen 

 be called i and that of the larger one J, looking from the photo- 

 meter, the ratio of brightness of the two is 



i.a e 



H -JT^ 



The same ratio of brightness of the two screens may be calcu- 

 lated from the angle of neutralization v, read off on the photo- 

 meter, from the formula 



R = C.tmf, 



in which C is a constant depending on the position of the prisms, 

 which is unknown, and has therefore first to be determined by 

 experiment. We have thus now the equation 



* PoggendorfF s Annalen, vol. cxviii. p. 193. 



