12 S. P. Langley — The History of a Doctrine. 



imaginary, of magnetism and electricity. But why have re- 

 course to invisible agents ? Quod petis, hie est. i It is merely 

 the ambient AIE." 



The capitals are Leslie's own, but ere we smile with supe- 

 rior knowledge, let us put ourselves in his place, and then we 

 may comprehend the exultation with which he announces the 

 identity of radiant heat and common air, for he feels that he 

 is beginning a daring revolt against the orthodox doctrine of 

 caloric ; and so he is. 



The first five years of this century are notable in the history 

 of radiant energy, not only for the work of Leslie, and for 

 the observation by Wollaston, Ritter, and others, of the so- 

 called " chemical " rays beyond the violet, but for the appear- 

 ance of Young's papers, reestablishing the undulatory theory, 

 which he indeed considered in regard to light, but which was 

 obviously destined to affect most powerfully the theory of ra- 

 diant energy in general. 



We are now in the year 1804, or over a century and a 

 quarter since the corpuscular theory was emitted, and during 

 that time it has gradually grown to be an article of faith in a 

 sort of scientific church, where Newton has come to be looked on 

 as an infallible head, ar d his views as dogmas, about which no 

 doubt is to be tolerated ; but if we could go. back to Cam- 

 bridge in the year 1668, when the obscure young student, in 

 no way conscious of his future pontificate, takes his degree 

 (standing twenty-third on the list of graduates), we should 

 probably find that he had already elaborated and greatly im- 

 proved certain already current ideas into the undulatory the- 

 ory of light, which he at any rate promulgated a few years 

 later, and afterward, pressed with many difficulties, altered, as 

 we now know, to an emissive one. 



Probably, if we could have heard his own statement then, 

 he would have told how sorely tried he was between these two 

 opinions, and, while explaining to us how the wavering bal- 

 ance came to lean as it did, would have admitted, with the 

 modesty proper to such a man, that there was a great deal to 

 be said on either side. We may, at any rate, be sure that it 

 would not be from the lips of Newton himself that we should 

 have had this announced as a belief which was to be part of 

 the rule of faith to any man of science. 



But observe how, if science and theology look askance at 

 each other, it is still true that some scientific men and some 

 theologians have, at any rate, more in common than either is 

 ready to admit ; for at the beginning of this century Newton's 

 followers, far less tolerant than their master, have made out of 

 this modest man a scientific pontiff, and out of his diffident 

 opinions a positive dogma, till as years go on, he comes to be 



