Photometric Observations of the Sun and Sky. 



261 



partially completed by using the formula i = O494 (O991) e where e 

 is the distance traversed by the Sun's rays in the atmosphere for 

 different altitudes. This formula is parallel to the equation t=Ap f 

 used by Pouillet in his memoir on the Solar Heat (and can be found 

 in 'Taylor's Memoirs, 1 vol. 4, p. 49). 



N.B. In this table, in each observation the sensitised paper was 

 exposed at right angles to the Sun's rays : so that a different portion 

 of the sky was observed at each altitude. 



Table B. 

 Chemical Action of Sun and Sky. 



14. It must be carefully noted, with respect to these older observa- 



ions, that what I actually observed, was the number of seconds' 



cposure at each altitude necessary to produce a particular darkening 



the sensitised paper, viz., the shade produced by the constant 



idle at distance unity, in that particular paper ; the numbers printed 



