390 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 



that centre and H 2 into another focus, and by the thermo- 

 pile, 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 sub- 

 mit 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 in- 

 visible, and, on the other, the visible spectrum. Differ- 

 ent 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 thermo- 

 pile, such as is commonly used, is liable, under these cir- 

 cumstances, to give deceptive results, and any error in its 

 indications counts in a double manner. It not only di- 

 minishes the value of one spectrum, but it adds that dim- 

 inution to the value of the other. The force of this re- 

 mark will be understood by considering the best experi- 

 ments hitherto made on this subject, those of Dr. Tyndall, 

 as related in his " Heat a Mode of Motion" (London edi- 

 tion, 1870, p. 420, etc.). In the case of the electric light, 

 the result yielded by those experiments was that the 

 heat in the invisible is eio;ht times that of the visible 



O 



region. But had there been an error in estimating the 

 position of the extreme red by only two millimeters, so 

 much would have been taken from the invisible and add- 

 ed to the visible that they would have been brought to 

 equality, and then the slightest turn of the screw that 

 carried the pile towards the dark space would have 

 given a preponderance to the visible. It is obvious, 

 therefore, that there cannot be certainty in such meas- 



