108 Dr. J. W. Draper on the Distribution of 



This variable absorptive action of the atmosphere depends 

 partly on changes in the amount of water- vapour^ and partly on 

 the altitude of the sun. At midday and at midsummer it is at 

 a minimum. The disturbance is not merely a thermochrose ; for 

 both ends of the spectrum are attacked. It is a matter of com- 

 mon observation that the horizontal sun has but little photo- 

 graphic power^ owing to atmospheric absorption of the ultra- 

 violet rays; and under the same circumstances his heating- 

 power is diminished^ owing to the absorption of the ultra-red 

 rays. But if the day be clear and the sun's altitude be sufficient^ 

 the visible spectrum may be considered unaffected. 



It should be borne in mind that the envelopes of the sun him- 

 self exert an absorptive action, which is powerfully felt in the 

 ultra-violet region, as is indicated by the numerous fixed lines 

 crowded together in that region. The force of this remark will 

 be appreciated on examining the Plate above referred to, in the 

 Philosophical Magazine for May 1843. 



It seems, then, that all the conditions necessary for the solu- 

 tion of this problem will be closely approached if we make use 

 of prisms constituted of any substance which is completely colour- 

 less to the eye, and confine our measures to the visible spectrum^ 

 collecting all the radiations between the fixed line A and the 

 centre of the spectrum just beyond D into one focus, and all the 

 radiations between that centre and H"^ into another focus, and, 

 by the thermopile or any other suitable means, measuring the 

 heat of these foci. 



Such is the method I have followed in obtaining the measures 

 now to be presented ; but before giving them there are certain 

 preparatory facts which I wish to submit to the consideration of 

 the reader. 



(1) In the mode of experiment hitherto adopted no special 

 care has been taken to ascertain with accuracy the position of the 

 "extreme red;'' yet that is held to be the point from which on 

 one side we are to estimate the visible, and on the other the in- 

 visible spectrum. Different persons, perhaps because of a differ- 

 ent sensitiveness of their eyes, will estimate that position differ- 

 ently. The red light shades off gradually ; it is almost impossible 

 to tell where it really comes to an end. A linear thermopile, 

 such as is commonly used, is liable under these circumstances to 

 give deceptive results ; and any error in its indications counts in 

 a double manner : it not only diminishes the value of one spec- 

 trum, but it adds that diminution to the value of the other. The 

 force of this remark will be understood by considering the best 

 experiments hitherto made on this subject — those of Dr. Tyn- 

 dall in his ' Heat a Mode of Motion ' (London edition, 1870, 

 p. 420 &c.). In the case of the electric light, the result yielded 



