﻿294 M. H. Wild on the Absorption of Light by the Air. 



served in continuously dry weather, while increased transpa- 

 rency is regarded as a sign of approaching wet weather. 



With these facts and observations are connected the two prin- 

 cipal views which at different times have been propounded as to 

 the greater or less transparency of the air. A. De la Rive ascribes 

 the less transparency of the air in dry weather to the presence of 

 opaque dust and vegetable germs. If the air becomes moist by 

 south-west winds setting in, these corpuscles become more trans- 

 parent, and at the same time heavier, owing to the absorption of 

 aqueous vapour, so that they fall to the ground more rapidly, 

 which is more completely the case when the rain begins ; the air 

 thereby becomes purified and at the same time more transparent. 

 Marshal Vaillant, on the contrary, assigns as the chief reason 

 for the different degrees of transparency in the air in north-east 

 and south-westerly winds, the circumstance that during the pre- 

 valence of the latter, owing to the uniformity in the temperature 

 of the ground and the air, local vertical currents are far less than 

 with north-easterly winds. Disturbed air, however, is far more 

 opaque, because at the boundary of warmer and colder layers of air 

 multifold reflections and irregular refractions of light take place. 



We will add no new hypotheses to these, but at once inquire 

 how far this question as to the various degrees of transparency 

 of the air and the absolute magnitude of the absorption of light 

 may be experimentally answered. 



Saussure was the first who attempted to measure the trans- 

 parency of the air. He devised for this purpose a simple instru- 

 ment, which he called the diaphanometer* . It consists of a black 

 circle in the centre of a white circle of three times its diameter. 

 In determining the transparency of the air two such disks are 

 necessary, one of which has a considerably greater (twelve times, 

 for instance) diameter than the other. If either of these disks be 

 gradually moved away from the eye, a point is ultimately reached 

 at which the black central spot disappears. This will be the 

 case when the visual angle of the black circle has become less than 

 the limiting angle of distinct vision, which amounts to about 50". 

 If we were merely concerned with this limiting angle, the black 

 spot of the larger disk would obviously disappear at twelve times 

 the distance of the smaller disk from the observer. If, then, as is 

 actually the case, the spot of the larger disk disappears at even 

 a smaller distance, the brightness of the white background has an 

 influence on this disappearance, and the deviation in the ratio of 

 the two distances at which the black circles of the smaller and the 

 larger disks disappear, from the ratio I : 12, may serve as a mea- 

 sure for the transparency of the air. From known photome- 

 trical principles, the two white disks uniformly illuminated by 

 * Memoires de V Academic de Turin, vol. iv.(l/89) p. 425. 



