ON THE THEORY OF LIGHT AND COLOURS. 



631 



fonly ; that light and heal occur to us, each 

 in two predicanients, ilie vibiatoiy or per- 

 manent, and the undulatory or transient 

 state; vibratory light being the minute mo- 

 tion of ignited bodies, or of solar phospbori, 

 and undulatory or radiant light, tbe motion 

 of the ethereal medium excited by these vi- 

 brations; vibratory iieat being a motion, to 

 M'hich all material substances are liable, 

 and w hich is more or less permanent ; and 

 iindnlHtory heat that motion of the same 

 ethereal medium, which has been shown by 

 Hoffmann, Buftbn, Mr. King, and M. Pic- 

 tet, to be as capable of reflection as light, 

 ind by Dr. Herschel to be capable of sepa- 

 rate refraction. (Phil. Trans. 1800. 284.) 

 How much more readily heat is communi- 

 cated by the free access of colder substances, 

 than either by radiation or by transmission 

 through a ([uiescent medium, has been shown 

 by the valuable experiments of Count llum- 

 ford. It is easy to conceive that some sub- 

 stances, permeable to liglit, may be unfit for 

 the transmission of heat, in the same manner 

 as particular substances may transmit some 

 kinds of light, while they are opaque with . 

 respect to others. 



On tiie whole it appears, that the few op- 

 tical phenomena which admit of explanation 

 by the corpuscular system, are equally con- 

 sistent with this tlieory ; that many others, 

 which have long been known, but never 

 understood, become by these means perfectly 

 intelligible ; and that several new facts are 

 found to be thus only reducible to a perfect 

 analogy with other facts, and to the simple 

 principles of the undulatory system. It is 



presumed, that henceforth the second and 

 third books of Newton's 0[)tics will be con- 

 sidered as more fully understood than the 

 first has hitherto been ; but, if it should ap- 

 pear to impartial judges, that additional 

 evidence is wanting fur the establishment of 

 the theory, it will be easy to enter more mi- 

 nutely into the details of various experiments, 

 and to show the insuperable dilhculties at- 

 tending the Newtonian doctrines, which, 

 without necessity, it would be tedious and in- 

 vidious to enumerate. The merits of their 

 author, in natural philosophy, are great be- 

 yond all contest or comparison ; his optical 

 discover}' of the com])osition of white light 

 would alone have immortalised his name ; 

 and the very arguments, which tend to over- 

 throw his s}'stem, give the strongest proofs 

 of the admirable accuracy of his experi- 

 ments. 



Sufficient and decisive as these arguments 

 appear, it cannot be superfluous to seek for 

 further confirmation ; which may with con- 

 siderable confidence be expected, from an 

 experiment very ingeniously suggested by 

 Professor Robison, on the refraction of the 

 light returning to us from the opposite mar- 

 gins of Saturn's ring ; for, on the corpuscu- 

 lar theory, the ring must be considerably 

 distorted when viewed through an achroma- 

 tic prism : a similar distortion ought also to 

 be observed in the disc of Jupiter ; but, if it 

 be found that an equal deviation is produced 

 in the whole light reflected from these planets, 

 there can scarcely be any remaining hope to 

 exjdain the aflTections of light, by a compa- 

 rison with the motions of projectiles. 



