412 



RADIATION OF HEAT. 



spectrum, but the diminution of its influence to this point, is gradual ; and it 

 cannot be inferred with certainty, that it might not exist in less degree in the ( 

 rays below this limit, and even in those invisible rays which are beyond the 

 red ray, unless we assume (hat there are no tests of chemical influence of 

 greater sensibility than those which have been used by the philosophers who ,' 

 instituted experiments on this subject. 



The presence of the luminous quality is determined by its effect on tho hu- 

 man eye, and the discovery of it must, therefore, be limited to the sensibility 

 of that organ. To pronounce that there are no luminous rays beyond the lim- 

 its of the visible spectrum, is to declare that the sensibility of the human eye 

 is infinite. Now, it is notorious, not only that the sensibility of sight in dif- 

 ferent individuals is different, but even that the sensibility of the eye of the ( 

 same person at different times, is susceptible of variation. If a person pass 

 suddenly from a strongly-illuminated apartment into a chamber, the windows 

 of which are closed, he will be immediately impressed with a sensation of ut- 

 ter darkness, and will be totally unable to discover any object in the room ; but 

 when he has remained some time in the darkened room, he will begin to be 

 sensible of the presence of light, and will, at length, even discern distinct ob- 

 jects. In this case, the eye, while exposed to the intense light of the first 

 chamber, accommodated its powers to the quantity of light to which it was ex- 

 posed, and, by a provision of nature, limited its sensibility in proportion as the 

 light was abundant. Passing suddenly into the darkened chamber, where a 

 very small quantity of light was admitted through the crevices of the windows, 

 the eye was incapable, in its actual state, of any perception of light, notwith- 

 standing the undoubted presence of that physical principle ; but when time 

 was allowed for the organ to adapt itself to the new circumstances in which it 

 was placed, its sensibility was increased, and a distinct perception of light ob- 

 tained. 



It is, therefore, perfectly certain, that the sensibility of the eye is variable 

 in the same individual, and even changeable at will. It is likewise perfectly 

 certain, that different individuals have different sensibilities of sight, one indi- 

 vidual being capable of perceiving light which is not visible to another. Cir- 

 cumstances render it highly probable that many inferior animals have a sensa- 

 tion of light, under circumstances in which the human eye has no perception 

 of it ; and it is, therefore, consistent with analogy to admit, at least, the possi- 

 bility, if not the probability, that the invisible rays which fall on the space be- 

 yond each extremity of the luminous spectrum, may be of the same nature as 

 the other rays of light, although they are incapable of exciting the retina of 

 the human eye in a sufficient degree to produce sensation. This, probably, 

 will receive still further support and confirmation, if we can show that these 

 nvisible rays enjoy all the optical properties, save and except that of affecting 

 he sight, which other luminous rays possess. 



It has already appeared that the non-luminous calorific rays, H, fig. 2, are re- 

 racted by transparent media in different degrees ; this refraction is also proved 

 o be subject to the same laws as the refraction of luminous rays. Thus the 

 sine of the angle of incidence bears a constant ratio to the sine of the angle 

 of refraction, when the refracting medium is given, and refracting media of dif- 

 erent kinds refract these rays in different degrees. 



If the invisible calorific rays at H, fig. 3, be allowed to pass through a hole 

 n the screen, and be received on the plane reflector M, they will be reflected in 

 he direction M H, in the same manner as a ray of light would be under the 

 same circumstances ; that is, the rays M H' and M H will be equally inclined 

 o the plane of the reflector. If rays of heat be received on a concave reikc 

 or, they will be reflected to a focus in exactly the same manner as rays of 



