514? Dr. Herschel's Experiments on the solar, and 



instance, a red ray is supposed to be twice as heating as a green 

 one, there will only go half the number of them to make up a 

 certain effect of heat ; and, on the other hand, if a green ray 

 should have a double power of illuminating, there will be no 

 more than half the number of them necessary to occasion a 

 certain effect of light. But, by my former experiments,* a red 

 ray, though much inferior to a green one, is probably fully equal 

 in illumination to a mean ray of all the colours united together. 

 Now, as red rays have also been proved to be accompanied by 

 the greatest heat, and as our bluish-white glass stops hardly 

 any invisible heat rays, we have certainly gone the full length 

 of fair concessions, by allowing all the light stopped by this 

 glass to be of that sort ; and thus it seems to be evident, that 

 the heat which lies under the colours, if I may use this expres- 

 sion, may be stopped, without stopping the colours themselves, 



It will not be necessary to lay much stress on this single 

 experiment; our second table affords us sufficient ground on 

 which to rest more forcible arguments. A dark-red glass, for 

 instance, was found to stop 606 rays of heat, and 999,8 of light. 

 This, even at the very first view, seems to amount to a total 

 separation of the two principles ; but let us discuss the pheno- 

 menon with precision. 



As only one ray in 5000 can make its way through this 

 glass, it is evident, that if the rays of light be also those of heat, 

 there can hardly come any heat through it but what must be 

 occasioned by rays that are invisible. It will therefore become 

 a question to be examined, how many of this sort we can ad- 

 mit, if we proceed on a supposition that heat consists of light, 

 as far as that will go. Now this, we find, has already been 



* See page 270, 10th experiment; 



