Wave-lengths in the Invisible Prismatic Spectrum. 213 



C represents the distribution according to a proposal (<#=log X) 

 of Lord Bayleigh. 



Curve D (j/ = C) is quite different from any of the pre- 

 ceding. It gives the distribution on a scale I have never 

 seen proposed, but which I have found useful. In this, the 

 bounding curve is a straight line parallel to the axis of X. 

 This construction is not well suited to exhibit the cold bands ; 

 but if we consider only the general distribution of the energy, 

 we shall find that curve D is not merely suggestive as illus- 

 trating what has already been remarked here as to the con- 

 ventional character of the methods of showing this distribution, 

 but that it has more practical uses ; for in this last construction 

 it is easily seen that the sums of the energies between any two 

 wave-lengths whatever are directly proportional to the dis- 

 tance between their ordinates, measured on the axis of X. If, 

 then, we desire (for instance) to know what relation the 

 invisible bears to the visible heat, or to inquire about what 

 point in the spectrum the energy is equally distributed, these 

 and similar problems are solved through curve D by simple 

 inspection. 



I have not been able yet to repeat the preceding determina- 

 tions upon the lower part of the spectrum as often as I could 

 wish. They are susceptible of improved accuracy by still 

 longer experiment, but I think that, within the limits of error 

 indicated, they may already be useful. I should add that 

 throughout this investigation I have received constant and 

 valuable aid from Mr. J. E. Keeler, not only in the graphical 

 constructions, but in the experiments and in the computations, 

 through all the details of which his aid has been more that of 

 a coadjutor than an assistant. 



Allegheny Observatory, Allegheny, Pa- 

 October 1883. 



Note. — Since the above was in type I have seen the inter- 

 esting article by M. H. Becquerel in the Annales de Chimie 

 for September 1883. 



The wave-lengths assigned by M. Becquerel to the band at 

 the limit of his researches at 1^460 to 1^480 appear to me 

 too great, for this limit corresponds to the band whose wave- 

 length is given at F'36 to 1^37 on my chart, published in the 

 Comptes Rendus of the previous year (September 11, 1882), 

 and on a larger scale in this Journal for March 1883, and 

 in the Annales de Chimie for August 1883. I regret 

 that M. Becquerel has not read the article in the Comptes 

 Rendus. Had he done so he would have seen that the 



