166 «od. W. Draper—Distribution of Heat in the Spectrum. 
the radiations between that center 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 meas- 
ures now to be presented: but before giving them there are cer- 
tain preparatory facts which I wish to submit to the considera- 
tion 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 invisible and on the other 
the visible spectrum. Different persons, perhaps because of a 
different sensitiveness of their eyes, will estimate that position 
differently. The red light shades off gradually—it is almost 
impossible to tell when it really comes to an end. <A linear 
thermopile, such as is commonly used, is liable under these cit 
cumstances to give deceptive results, and any error in its indi- 
cations counts in a double manner. It not only diminishes the 
value of one spectrum, 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. Tyndall, as related in his “Heat a Mode 0 
Motion” (London edition, 1870, p. 420, &c.). In the case of 
the electric light, the result yielded by those experiments was 
that the heat in the invisible is eight times that of the visible 
region. But had there been an error in estimating the post 
tion of the extreme red by only two millimetres, so much would 
have been taken from the invisible and added to the visible, 
that they would have been brought to equality, and then the 
slightest turn of the screw that carried the pile toward the dark 
space would have given a preponderance to the visible. It's 
obvious, therefore, that there cannot be certainty in such meas 
ures, unless the fixed lines are resorted to as standard points. 
2.) A ray which has passed through a solution of sulphate 
of copper and ammonia possesses no insignificant heating powe® 
I took a stratum of a solution of that salt, of such strength that 
it only permitted waves to pass which are of less length than 
4860. Seen in the spectroscope, the colors transmitted through : 3 
it commenced with a thin green fringe, followed by blue, ind .. 7 
. 
violet. It therefore gave rays in which, according tO ag 
accepted views, little or no heat should be detected. Yet 
found that such rays produced one-ninth of the heat of the solar : a 
eam. Does not this indisputably show that the more relay 
gible rays have : higher calorific power than is commonly : 
imputed to the 
8.) Again, by the use of the apparatus presently 10 > 
be 
deseri d, I found no difficulty in recognizing heat in the violet Le 
? 
