402 Sir Johri F. W. Herschel on the Absorption 



in this respect as we proceed from one end of the spectrum to 

 the other. When we contemplate the subject in this point of 

 view, all idea of regular functional gradation is at an end. We 

 seem to lose sight of the great law of continuity, and to find 

 ourselves involved among desultory and seemingly capricious 

 relations, quite unlike any which occur in other branches of 

 optical science. It is, perhaps, as much owing to this as to 

 anything, that the phaenomena of absorption, in some recently 

 published speculations, and in the view which Mr.Whewell 

 has taken in his Report of the progress and actual condition of 

 this department of natural philosophy, read to this Meeting, 

 have been characterized as peculiarly difficult to reconcile 

 with the undulatory theory of light. In so far as I have above 

 described the phaenomena in appropriate terms, it will be evi- 

 dent that a certain difficulty must attach to their reduction 

 under the dominion of any theory, however competent, ulti- 

 mately, to render a true account of them. Where such evi- 

 dence of complication and suddenness of transition subsists on 

 the face of any large assemblage of facts, we are not to expect 

 that the mere mention of a few general propositions, like ca- 

 balistic words, shall all at once dissipate the complication, and 

 render the whole plain and intelligible. If we represent the 

 total intensity of light, in any point of a partially absorbed 

 spectrum, by the ordinate of a curve whose abscissa indicates 

 the place of the ray in order of refrangibility, it will be evi- 

 dent, from the enormous number of maxima and minima it 

 admits, and from the sudden starts and frequent annihilations 

 of its value through considerable amplitudes of its abscissa, 

 that its equation, if reducible at all to analytical expression, 

 must be of a singular and complex nature, and must at all 

 events involve a great number of arbitrary constants dependent 

 on the relation of the medium to light, as well as transcendents 

 of a high and intricate order. We must not, therefore, set it 

 down to the fault of either of the two rival theories if we do 

 not at once perceive how such phaenomena are to be reconciled 

 to the one or to the other, but rather endeavour to satisfy our- 

 selves whether there be, in the first instance, anything in the 

 phaenomena, generally considered, repugnant either to sound 

 dynamical principles, or to the notions which those theories 

 respectively involve as fundamental features. 



Now, as regards only the general fact of the obstruction and 

 ultimate extinction of light in its passage through gross media, 

 if we compare the corpuscular and undulatory theories, we 

 shall find that the former appeals to our ignorance, the latter 

 to our knowledge, for its explanation of the absorptive phae- 

 nomena. In attempting to explain the extinction of light, on 



