Prof. Draper on the Diffraction Spectrum. 155 



media, endeavouring to get rid of all the distui-bances which 

 arise by the absorptive action of glasses, by using as the grating 

 a polished surface of steel, on which parallel lines were drawn 

 with a diamond, and employing a concave mirror instead of an 

 achromatic lens; and although my results were imperfect and 

 incomplete, I saw enough to convince me that this is the right 

 course to be taken in investigating the problem. It is absolutely 

 necessary to employ a spectrum which has been formed by 

 reflexion alone. 



I will also add, that, in the experiments here referred to, a 

 method was resorted to for determining the temperatures of nar- 

 row spaces, which may be recommended to those who are disposed 

 to resume these inquiries. It is to employ a blackened platinum 

 wire I of an inch long and ^^yth of an inch in diameter, one end 

 of which is fastened to the end of a bar of bismuth, the other 

 end to the end of a bar of antimony, each of these bars being 

 f of an inch square on the end, and 4 inches long, their distant 

 extremities communicating with a galvanometer. By carrying 

 the platinum wire transversely along the spectrum, I expected not 

 only to determine the distribution of heat, but also to ascertain 

 whether the heat-spectrum has fixed lines like the luminous and 

 chemical ones. 



This was at a time when I first began to suspect that the 

 essential difi^erence between Hght and heat is this: viz. that 

 whilst the vibrations for light are transverse, those for heat are 

 normal, and its waves in that respect analogous to the waves of 

 sound J but so feeble is the intensity of these spectra, that I 

 could do no more than satisfy myself that in the diffi-action 

 spectrum the centre of the yellow is really the hottest space, as 

 well as the most luminous. I believe that there is a cold line in 

 the spectrum answering in position to the dark line H, but I 

 could not absolutely demonstrate it. 



Nevertheless so certain does it appear that the distribution of 

 heat corresponds to the distribution of light, that I have not 

 hesitated since that time to look upon the centre of the yellow 

 space as the point of maximum heat, from which there is a 

 decline to each end of the spectrum. And I accordingly made 

 this the basis of the theory of vision in my ' Treatise on Human 

 Physiology,' published last year. Those who are interested in 

 these subjects will find in that work a discussion of the whole 



?uestion, founded upon the phsenomenaof the diffraction spectrum, 

 n the English edition, published by S. Low and Son, London, 

 it is on pages 386-399. Such a view has the advantage of being 

 sustained by many facts of comparative anatomy. It obliges us, 

 it is true, to return to the opinions entertained of the functions 

 of the black pigment more than a century ago ; but then it gives 



M2 



