100 THE DIFFRACTION SPECTRUM. [MEMOIR VI. 



tion of heat, but also to ascertain whether the heat spec- 

 trum has fixed lines, like the luminous and chemical ones. 



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

 the essential difference between light and heat is this, 

 viz., that while the vibrations of light are transverse, 

 those of heat are normal, and its waves in that respect 

 analogous to the waves of sound ; but so feeble is the 

 intensity of these spectra that I could do no more than 

 satisfy myself that in the diffraction 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 distri- 

 bution 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 a 

 theory of vision, published in my Treatise on " Human 

 Physiology." 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 cen- 

 tury ago; but then it gives a very elegant explanation 

 of the uses of the different parts of the retina, examined 

 in its radial section after the manner of M. H. Muller, 

 and of those of the choroid coat structures which are 

 without meaning on the ordinary theory of vision. 



UNIVERSITY OF NEW YORK, Jan. 26, 1857. 



NOTE. To the foregoing paragraphs I may add 

 the following as contributions to the theory of vision. 



