on the Optical Phcenomena of the Atmosphere. 101 



On the Grossglockner also, by observing the zenith and the low- 

 est visible point of the horizon, we found a striking coincidence 

 with the law above mentioned; we there found — 



But between these limits the increase was not quite so regular. 

 Thus we found at a zenith distance of 50°, 70 per cent, of cobalt, 

 whereas the calculation gives only 59 ; an error, however, which 

 at this altitude scarcely exceeds that due to a single degree of 

 Saussure's cyanometer. When the firmament is observed from 

 deep valleys, the lateral intensity of the blue colour is very irre- 

 gular. In this case local vapours amass themselves, and cause 

 the observed depth of hue to be much less than that obtained 

 from calculation. 



Pursuing the method already described, we have also attempted 

 to ascertain the quantity of yellow and reddish colour (ochre) 

 which enters into the composition of the sky. The quantity of 

 the latter naturally depends on the colour of the cobalt and 

 white, as these themselves are not absolutely pure colours. This 

 combination of more than simple blue and white has also the 

 advantage, that by it we are enabled to determine the brightness 

 itself with greater certainty. The coincidence of shade between 

 the actual and the artificial colours greatly facilitates the com- 

 parison of both. 



Table of Observations ivith the Tricoloured Cyanometer (No. II.). 



