174? TH/iORY OF LIGHT AND COLOURS. 



account in a fatisfactory manner for all the phenomena. But, 

 contrarily to what might have been expected from Newton's 

 ufuaJ accuracy and candour, he has laid down a new law for 

 the refraction, without giving a reafon for rejecting that of 

 Huygens, which Mr. Hauy has found to be more accurate than 

 Newton's; and, without attempting to deduce from his own 

 fyftem any explanation of the more univerfal and linking ef- 

 fects of doubling fpars, he has omitted to obferve that Huy- 

 gens's mod elegant and ingenious theory perfectly accords with 

 thefe general effects, in all particulars, and of courfe derives 

 from them additional pretentions to truth : this he omits, in 

 order to point out a difficulty, for which only a verbal folu- 

 tion can be found in his own theory, and which will probably- 

 long remain unexplained by any other. 

 MicheiPs expe- o. Mr. Michell has made fome experiments, whioh appear 



momentum of to mow ^ iat tne ra y s °f n S nt nave an a & ua l momentum, by 

 lightexphined, means of which a motion is produced when they fall on a thin 

 ZSfjT *ni-l P Iate of co PF er delicately fufpended. (Priefliey's Optics.) 

 tion. But, taking for granted the exact perpendicularity of the plate, 



and the abfence of any afcending current of air, yet fince, in 

 every fuch experiment, a greater quantity of heat mult be 

 communicated to the air at the furface on which the light falls 

 than at the oppofite furface, the excefs of expanfion mutt ne- 

 ceffarily produce an excefs of preffure on the firft furface, and 

 a very perceptible receflion of the plate in the direction of the 

 They did not light. Mr. Bennet has repeated the experiment, with a 

 fucceed with mU ch more fenfible apparatus, and alio in the abfence of air ; 

 and very juftly infers from its total failure, an argument in fa- 

 vour of the undulatory fyftem of light. (Phil. Tranf. for 17.92, 

 p. 87.) For, granting the utmoft imaginable fubtiJity of the 

 corpufcles of light, their effects might naturally be expected to 

 bear fome proportion to the effects of the much Iefs rapid mo- 

 tions of the electrical fluid, which arc fo very eaiily percep- 

 tible, even in their weakeft ftates. 

 latent light and 3. There are fome phenomena of the light of folar phof- 

 fiftTnrwkh C thc" P bon *' which at firft fight might feem to favour the corpufcular 

 do&rineofvi- fyftem; for inftance, its remaining many months as if in a 

 brations. latent ftate, and its fubfequent re-emiffion by the action of 



heat. But, on further confideration, there is no difficulty in 

 fuppofing the particles of the phofphori which have been made 

 to vibrate by the action of light, to have this action abruptly 



fufpended 



