56 WAVE LENGTHS OF THE SEVEN GREAT RAYS. 



197. The interference spectrum is given iujlg. 133 and in the frontispiece, with its 

 fixed lines and coloured spaces, as seen on the ground glass. By the side of it is placed the 

 prismatic spectrum, the spectrum of NEWTON. Compared together, we see that in the 

 latter the yellow region is among the less refrangible rays, in the former it occupies 

 the central position of the spectrum. The distribution of the fixed lines is also differ- 

 ent, though their position, as respects the coloured spaces, is the same for both spectra. 

 Of course the light is distributed differently in respect to its intensity in the two spec- 

 tra, as has been said : for the one it is a maximum in the centre, and fades away equally 

 to each extremity ; but for the other, the illuminating power is as given in the table 

 (147) derived from FRAUNHOFER. 



198. The following table, derived from Sir J. HERSCHEL'S Treatise on Light, gives 

 the lengths of the waves of light corresponding to each one of the principal lines of the 

 spectrum. The Paris inch is supposed to be divided into one hundred millions of equal 

 parts, one of which constitutes the unit of the scale. 



Length of wave corresponding to the ray B, 2541 

 " " " C, 2422 



D, 2175 



" " E, 1945 



" " F, 1794 



* " " G, 1587 



* " H, 1464 



199. In the preceding chapter, we have given a brief account of the experiments of 

 M. MELLONI on the distribution of heat in the spectrum, and shown (170) bow he de- 

 duced from those results the doctrine of the physical independence of the two im- 

 ponderable principles. More recently, however, M. MELLONI has repeated the same 

 experiments (Comptes Rendus, Jan., 1844), and draws from them a result diametrically 

 opposed to the former. 



200. That a philosopher should, with the progress of knowledge, change his opin- 

 ions on fundamental points of science, is very far from being a fault. For more than 

 ten years M. MELLONI has studied, with unexampled success, the laws and phenomena 

 of radiant heat. The leading doctrine which he has deduced from those researches is 

 that to which we have just referred, the entire physical independence of light and heat. 

 From recent repetitions of his former experiments, he has now arrived at a conclusion 

 apparently opposite, " that light is only a series of calorific indications sensible to the 

 organ of sight, or, vice versa, that the radiations of dark heat are true invisible radia- 

 tions of light." The arguments which have been employed (169) in discussing the 

 experiments of SEEBECK and others, on the shifting of the maximum point when dif- 

 ferent diaphanous media are used from the red into the orange, or even into the yellow, 

 all hang upon the admission that the spectrum under consideration is sufficiently pure, 

 and that there is no superposition or overlapping of its parts. For, were this the case, 

 even where colourless and diaphanous media are used, such as crown-glass, water, sul- 

 phuric acid, and alcohol, though these, by reason of their transparency, do not exert 

 any absorbent action on the rays of light, they might, by reason of their invisible col- 



