" On the Absorption of Specific Rays, <$-c" 423 



I do not think that absorption is to be considered a neces- 

 sary part of the theory. It is a sort of extraneous interruption 

 which either leaves the ordinary laws in full vigour, or wholly 

 destroys, not the laws, but that which is the subject of the laws. 

 Reflexion, refraction, interference, double refraction, polari- 

 zation, go on with absorption just as if there were no such 

 thing in nature. The supposition of undulation by transver- 

 sal vibration, the principle of superposition of small vibrations, 

 the assumption that the velocity is different in different media, 

 are necessary in every investigation ; the suppositions (what- 

 ever they are,) that are to account for absorption are necessary 

 ontynow and then. The former suppositions, in the vast ma- 

 jority of instances, do not require the latter; the latter when 

 wanted must be combined with the former. These considera- 

 tions seem to point out clearly that absorption requires a snp- 

 plementary theory ; and our only care with optical theories at 

 present must be, that our present assumptions may admit of 

 such a supplement at some future time. As far as I can judge, 

 either theory (emission or undulation) seems likely to admit of 

 such a supplement, and I do not see that one will admit of it 

 more easily than the other. 



A remarkable instance of the same kind has already oc- 

 curred in the history of optics. When Fresnel's theory and 

 measures of diffraction had given a very high probability to the 

 undulatory theory, there still remained, to be accounted for, 

 the laws of polarization and the connexion between polariza- 

 tion and double refraction. The undulatory theory was there- 

 fore generally adopted, leaving the kind of vibration to be deter- 

 mined by the consideration of accounting in the best manner 

 for these remaining phaenomena. The success with which 

 this was afterwards done, by the assumption of transversal 

 vibrations, exceeds anything that has been gained in philoso- 

 phy since the establishment of the theory of universal gravita- 

 tion. Had Fresnel proceeded as you (apparently) would wish 

 us to proceed, the undulatory theory would not now have ex- 

 isted. 



Every other branch of philosophy presents instances similar 

 to the last. If, for instance, at the time of inquiring into the 

 mutual action of bodies on each other, Newton had insisted 

 on including in his general theory (whatever it might be,) the 

 effects of what we now call magnetism and capillary attraction, 

 the theory of gravitation would never have been formed. By 

 leaving these as subjects for future investigators, and by re- 

 ducing to law the preponderating set of phaenomena, he was 

 able to form the most complete cosmical theory that has ever 

 appeared. Many years passed before those supplementary 



