868 S. P. Langley — Energy and Vision. 



of the selective absorption of silver there as an interesting field 

 still open for experiment to those engaged in the study of that 

 end of the spectrum. 



By means of this graphic construction, which is amply ac- 

 curate for the immediate purpose, and by the use of the for- 

 mula already described (Memoirs of the National Academy, 

 vol. ii, p. 161), we can also pass from the actually observed 

 prismatic spectrum to the effect which would have been ob- 

 served in a truly normal one ; and it is by the use of these 

 constructions, founded on these f ormuloe, - that the final reduc- 

 tions here given have been obtained. It is here assumed that 

 no sensible selective absorption is exerted by the prism or any 

 other portion of the apparatus. 



"We now give a summary of the photometric observations. 

 The state of sky for each series and the approximate air masses 

 were: — 



March 30th.— Sky " fair blue;" observer, S. P. L. ; time, ll h 40 m 

 a. m. to 12 h 45 Am p. m. (Greenwich 5th hour meridian time); air 

 mass,* 1*22 atmospheres. 



April 2d. — Sky " milky blue with cumuli ;" observer, F. W. V.; 

 time 12 h to 2 h p. m.; air mass, 1*19 atmospheres. 



April 3d. — Sky " blue with cumuli," better than on 2d, sky 

 better in E. M.'s series than in that of F. W. V., when a slight 

 haze, barely perceptible, had formed. 



1st series; observer, E. M. ; time, ll h 10 m a. m. to 12 b 30 m p. m.; 

 air mass, 1*18 atmospheres. 



2d series ; observer, F. W. V. ; time, l h 15 m to 2 h 30 m p. m. ; air 

 mass, 1*28 atmospheres. 



April 4th. — "A good blue at first, after 12 h milky blue from 

 slight smoke, but still a fairly good sky." 



1st series; observer, F. W. V. ; time, 10 h 25 m to ll h 55 m A. m. ; 

 air mass, 1*23 atmospheres. 



2d series; observer, E. M. ; time, 12 a 30 m to l h 30 m p. m. ; air 

 mass, 1'20 atmospheres. 



April 6th. — Sky "good blue, quite clear;" observer, F. W. V. ; 

 time, 10 h 45 m a. m. to 12 h 15 m p. m. ; air mass, 1-18 atmospheres. 



June 16th. — Sky " clear, good blue ; after 1 p. m. cirrus streaks. 

 Observer, B. E. L. ; time, ll h 15 m a. m. to l h 45 m p. m. ; air mass, 

 1*05 atmospheres. 



July 2d. — Sky " clear, excellent ;" observer, B. E. L. 



1st series; time, 1 l h 25 m a. m. to 12 h 15™ p. m.; air mass, 1.03 

 atmospheres. 



2d series; time, 12 h 15 m p. m. to l h 25 m p. m.; air mass, 1*02 

 atmospheres. • 



* By air mass is here meant that actually traversed by the solar rays, that with 

 a vertical sun at sea level being unity. 



