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



their opinions ready-made as to science in general ; each know- 

 ing, so far as he can be said to know anything at first-hand, 

 only that little corner which research has made specially his 

 own. 



The moral we can all draw, I think, for ourselves. 



In spite of such criticism as this, the nndulatory hypothesis 

 of light made rapid way, and carried with it, one would now 

 say. the necessary inference that radiant heat was due to un- 

 dulations also. This was, however, no legitimate inference to 

 those to whom radiant heat was still a fluid ; and yet, in spite 

 of all, the modern doctrine now begins to make visible 

 progress. 



A marked step is taken about 1811 by a young Frenchman, 

 De la Eoche, who deserves to be better remembered than he is, 

 for he clearly anticipated some of Melloni's discoveries. De la 

 Roche in particular shows that of two successive screens the 

 second absorbs heat in a less ratio than the first ; whence he, 

 before any one else, I believe, derives the just and most im- 

 portant, as well as the then most novel conception, that radi- 

 ant heat is of different kinds. _ He sees also, that, as a body is 

 heated more aud more, there is a gradual and continual ad- 

 vance not only in the amount of heat it sends out, but in the 

 kind, so that, as the temperature still rises, the radiant heat 

 becomes light by imperceptible gradations ; and he concludes 

 that heat and light are due to one simple agent, which, as the 

 temperature rises yet more, appears more and more as light, or 

 which, as the luminous radiation is absorbed, re-appears as 

 heat. Yery little of it, he observes, passes even transparent 

 screens at Low temperatures, but more and more does so as the 

 temperature rises. 



All this is a truism in 1888, but it appears admirably new as 

 well as true in 1811 ; and if De la Roche had not been re- 

 moved by an early death, his would have not improbably been 

 the greatest name of the century in the history of our subject ; 

 an honor, however, which was in fact reserved for another. 



The idea of the identity of light and radiant heat had by 

 this time made such progress that the attempt to polarize the 

 latter was made in 1818 by Berard. We have just seen in 

 Herschel's case how the most sound experiment may lead to a 

 wrong conclusion, if it controvert the popular view. We now 

 have the converse of this in the fact that the zeal of those who 

 are really in the right way may lead to unsound and inconclu- 

 sive experiment ; for Berard experimentally established, as it was 

 supposed, the fact that obscure radiant heat can be polarized. So 

 it can, but not with such means as Berard possessed, and it was 

 not till a dozen years more that Forbes actually proved it. 



