Prof. S. P. Langley on Energy and Vision. 11 



is referred to the footnote.* It will be seen from this table 

 that while such a silver film exercises a considerable selective 

 absorption in the ultra-violet, and even at the blue end, it 

 exercises less as the wave-length increases, and in fact an 

 extension of it would show a still enhanced power of reflexion 

 for infra-red rays. It is with these infra-red rays that our 

 measurements in previous researches on radiant heat at this 

 Observatory have been hitherto mainly made. Accordingly 

 our measurements of the selective reflexion in the ultra-violet, 

 to which we have given comparatively little study, have not 

 been repeated with all the care which the subject deserves, 

 and we recommend a more complete determination 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 

 accurate for the immediate purpose, and by the use of the 

 formula already described (in 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 observed in a truly normal one ; and it is by the use of 

 these constructions, founded on these formulae, that the final 

 reductions 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, 

 Wh. 40"^ A.M. to 12^ 45°^ P.M. (Greenwich 5th hour meridian 

 time) ; air-masst, 1*22 atmospheres. 



April 2nd. — Sky "milky blue with cumuli;" observer, 

 F. W. V. ; time, 12^" to 2^ p.m. ; air-mass, 1"19 atmospheres. 



April 3rd. — Sky " blue with cumuli,^' better than on 2nd, 

 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, 11^ 10°^ a.m. to 12^ 30^" 

 P.M. ; air-mass, 1"18 atmospheres. 



* The selective absorption of silver has been deduced by bolometric 

 measurements in the solar spectrum, ■with a Rutherfurd grating, by pro- 

 ducing multiplied successive reflexions of the light from silver before 

 allowing it to enter the slit of the spectroscope and determining succes- 

 sively the variation in the intensity of different rays according to the 

 number of reflexions. The observations are reduced by a logarithmic 

 formula. 



t By air-mass is here meant that actually traversed by the solar rays, 

 that with a vertical sun at sea-level beiug unity. 



