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XXIX. The History of the Doctrine of Radiant Energy. 

 By Lord Rayleigh, Sec. R.S.* 



IN his interesting Address f to the American Association 

 for the Advancement of Science, Prof. Langley sketches 

 the development of the modern doctrine of Radiant Energy, 

 and deduces important lessons to be laid to heart by all con- 

 cerned in physical investigation. This is a most useful under- 

 taking ; but in the course of it there occur one or two state- 

 ments which, in the interest of scientific history, ought not to 

 be allowed to pass without a protest. 



After quoting Melloni's very unequivocal conclusion of 

 1843, that " Light is merely a series of calorific indications 

 sensible to the organs of sight; or, vice versa, the radiations 

 of obscure heat are veritable invisible radiations of light," 

 Prof. Langley goes on to say, " So far as I know, no phy- 

 sicist of eminence reasserted Melloni^s principle with equal 

 emphasis till J. W. Draper, in 1872. Only sixteen years 

 ago, or in 1872, it was almost universally believed that there 

 were three different entities in the spectrum, represented by 

 actinic, luminous, and thermal rays." 



These words struck me strangely as I first read them. My 

 own scientific ideas were formed between 1860 and 1866, and 

 I certainly never believed in the three entities. Having on a 

 former occasion referred to this question % as an illustration 

 of the difference of opinion which is sometimes to be found 

 between the theoretical and experimental schools of workers, 

 I was sufficiently interested in the matter to look up a few 

 references, with results which are, I think, difficult to re- 

 concile with Prof. Langley^s view. 



In Young's Lectures§ we read: — "Dr. HerschePs experi- 

 ments have shown that radiant heat consists of various parts 

 which are differently refrangible, and that, in general, in- 

 visible heat is less refrangible than light. This discovery 

 must be allowed to be one of the greatest that have been made 

 since the days of Newton .... 



" It was first observed in Germany by Ritter, and soon after- 

 wards in England by Dr. Wollaston, that the muriate of silver 

 is blackened by invisible rays, which extend beyond the pris- 

 matic spectrum, on the violet side. It is therefore probable that 

 these black or invisible rays, the violet, blue, green, perhaps 



* Communicated by the Author. 



t Amer. Joum. Sci. Jan. 1889. 



X Address to Section A, Brit. Assoc. Report, 1882. 



§ Vol. i. p. 638 (1807). 



