360 ' On the Refraction of Light. 



an intermediate refraction, and must necessarily occupy, on the spec- 

 trum, a position between the red and the yellow. The violet rays, 

 being most rare, penetrate the thickest part of the prism, and con- 

 sequently, being most refracted, must occupy the summit of the spec- 

 trum. It would follow, from this theory, that a beam of white light, 

 in permeating a double convex lens, would undergo analysis, and 

 paint many spectra, on a white surface at some distance from its focus, 

 since the lens is of unequal thickness. Such is really the case, 

 though the colors produced cannot separately affect the retina, from 

 the circumstance of their being developed in so many points of 

 the lens, and blending so intimately together as to make white light. 

 But that light is decomposed by the lens, may be proved thus : 

 Attach to the back of the lens a disk of sheet lead having any num- 

 ber of punctures, and let a beam of light, entering a dark chamber, 

 strike on the lead ; at some distance beyond the focus, hold a sheet 

 of letter paper, and the image of each hole in the disk will be seen 

 occupied by a spectrum, glowing with the primitive colors. The 



other column was placed near it, and facing the mirror, to support an iron globe. 

 Things were so ordered that, on heating the iron ball, all the calorific rays, incident 

 on the mirror, would be reflected, and those only, which fell on the silver lens, 

 would be transmitted. On subjecting the globe to an incandescent heat, I held the 

 bulb of a thermometer in the spot where the focus would be, if a beam of light 

 were refracted through a diaphanous lens of the same dimensions; but the ther- 

 mometer did not there indicate a higher temperature than in any other spot, equi- 

 distant from the metallic lens and within the cone of radiation. Nor did the unequal 

 thickness of the lens appear to affect, in any degree, the intensity of the transmitted 

 heat. This experiment, which was cautiously performed, has convinced me ttiat 

 caloric, per se, is not subject to the laws of refraction. When united with light, 

 in a combination which may be called chemical, it accompanies its rays as a neces- 

 sary and indispensable agent. The combination of caloric with the red rays seems 

 to be in a certain invariable ratio above the surrounding temperature, and that ratio 

 is directly as the density of the colorific rays. It becomes thus, easy to account for 

 the inequality of heat presented on the spectrum, or transmitted through glasses of 

 different colors or lenses, when we reflect that caloric is held, chemically combined 

 with the atoms of each color of light, and the more the colorific rays are condensed, 

 the more sensibly will their caloric be developed. That heat should be found beneath 

 the apparent base of the spectrum may seem at first paradoxical ; but when it is 

 recollected, that the illuminating, or properly lucific rays, through which alone ob- 

 jects are visible, are totally distinct fi-om the colorific rays, the anomaly immediately 

 vanishes. For the densest rays may be collected where sensible caloric predomi- 

 uates, and still be invisible to us, from the circumstance of their not being combined 

 with the illuminating rays. That a fluid, like caloric, perfectly homogeneous in all 

 its parts, and producing uniformly the same effects, should consist of differently re- 

 frangible rays, is an hypothesis, so diametrically repugnant to all philosophical prin- 

 ciples, that it is wonderful it should have received a moment's consideration. 



