Heat in the Spectrum. 107 



it were possible to make satisfactory heat-measures on the dif- 

 fraction spectrum, in which the coloured spaces and fixed lines 

 are arranged according to their wave-lengths, the admission 

 would be substantiated. 



In view of these facts I did attempt many years ago to make 

 heat-measurements on the diffraction spectrum ; but so small 

 is the heat, that, as may be seen in the Philosophical Magazine 

 (March 1857), the results were unsatisfactory. More recently 

 I have tried another method of investigation, on'principles which 

 I will now explain. 



For the sake of clearness, restricting our thoughts for the 

 moment to the more familiar case of the visible spectrum, if we 

 desire to ascertain the true distribution of heat, would not the 

 proper method be to collect all the less-refrangible rays into one 

 focal group and all the more refrangible into another focal group, 

 and then measure the heat that each gave ? If the view cur- 

 rently received be correct, would not nearly all the heat observed 

 be found in the former of these foci, and little, if indeed any, be 

 found in the second ? But if all the various regions of the spec- 

 trum possess equal heat-giving powers, would not the heat in 

 each of these foci be the same ? ^ 



Let us give greater precision to this idea. Using Angstrom^s 

 wave-lengths, the length at the line A is 7601, that at H^ 3933 ; 

 and these lines are not very far from the less and more refran- 

 gible ends of the visible spectrum respectively. The middle point 

 of this spectrum is at 5768, which, therefore, may be called its 

 optical centre. This is a little beyond the sodium-line D, which 

 is 5892. Now, if by suitable means we reunite all the rays be- 

 tween 7604 and 5768 into one focus, and all the rays between 

 5768 and 3933 into another focus, are we not in a position to 

 determine the true distribution of the heat ? Should the heat 

 at these two foci be sensibly the same, must not the conclusion 

 at present held be abandoned ? 



If in these investigations the rays of the sun are used, it is 

 necessary to restrict the examination to the visible spectrum, 

 excluding the invisible red and invisible violet radiations. On 

 these the earth's atmosphere exerts not only a very powerful but 

 a very variable action, and what is still more, an action the result 

 of which we cannot see, so that we are literally working in the 

 dark. There are days on which, owing to the excessive absorp- 

 tion taking place among the ultra-red rays, a rock-salt train hts 

 no advantage over one of glass. But if it be the visible spec- 

 trum alone that we are using, and the prisms are of a material 

 colourless to the eye, we may be certain that they are exerting no 

 elective absorption on any of the radiations of that spectrum, 

 and that the indications they are giving are reliable. 



