14 Dr. H. Schlagintweit's Observations in the Alps 



higher we ascend, the more nearly does the atmosphere approach 

 the state of perfect transparency. When, however, the barometer 

 stood at 479 millims., a loss of light was still appreciable. The 

 degree of transparency during serene, and to all appearance, 

 perfectly clear days, is subject to variation ; which perhaps de- 

 pends upon general psychromctric circumstances, but more im- 

 mediately upon the condensation of atmospheric moisture occa- 

 sioned by the peculiarities of locality and temperature. To this 

 may be attributed the difference between the Wasseradkopf and 

 Adlersruhe. Water distributed throughout the atmosphere in a 

 gaseous form increases the transparency. It is known, for in- 

 stance, that the outlines of neighbouring mountains are pecu- 

 liarly visible immediately before the descent of rain. 



The greatest number of luminous rays are absorbed by the 

 atmosphere in the immediate vicinity of the source from which 

 they, either directly or by reflexion, proceed, — a law quite ana- 

 logous to that which, as before observed, Melloni discovered for 

 the rays of heat. The most evident case of this kind is obtained 

 from a comparison of Nos. 3 and 4. An object at a distance 



a of 229' disappeared under 1' 15" 

 b ... 2740' ... 1' 16" 



c ... 26100' ... 1' 19". 



The differences of the distances increase here far more quickly 

 than those of the angles. 



For a perfectly transparent atmosphere the quotients (column 

 8 of the table in page 13) would be =12. Calling this 1000, 

 we obtain for the quotients due to the respective altitudes the 

 following numbers : — 



On the Grossglockner .... 996 



^ ,, f Adlersruhe .... 991 



0n the iRachern 995 



On the Johannishiitte .... 981 



In Lienz 856 



Differences which are great enough to be the cause of consider- 

 able eri-ors in judging of distances at great heights, show them- 

 selves here. If objects, the size of which is approximately 

 known to us, as men, animals, houses, &c, be observed at great 

 elevations, we are generally induced to consider them nearer 

 than they really are. Objects which enable us to draw no con- 

 clusion as to distance, such as masses of projecting rock, &c, 

 appear to us too small. The reverse of this property of transpa- 

 rency to diminish the apparent size of bodies is exhibited when 

 the atmosphere is obscured by fog, &c. In this case mountain 

 summits are considerably elevated, and appear to us rougher and 



