I On the unequal Reftexibilttj of Light, 22 J 



In a word, therefore, the rays differ in refkxibility in the Newtonian fenfe, and the moft 

 refrangible is alfo the moft reflexible. 



Thus far, in order to give fimplicity to my argument, I have-left the refradling angle C, 

 of the prifm, undetermined. Newton determines it m the ninth experiment of the firft book 

 of his optics, part r. He ufed a refracting angle of 45°; and, neverthelefs, he fays ex- 

 prefsly, that the rays entered perpendicularly ; whence it follows, that the angle of incidence 

 at the point M was alfo 45°. We fhould, therefore, be juftified in thinking, that rays 

 falling under this angle of incidence on the furface B C, paffing from glafs to air, are not 

 totally refleiSed : Briflon affirms this to be the cafe, when he explains this fame experiment in 

 his Elementary Treatife of Natural Philofophy, printed at Paris, in the year 1789. vol, II. § 

 1411. Neverthelefs, it is well known that the total refledlion takes place at a fmaller 

 angle, that is to fay, about 40°. This has been determined, even with the utmoft precifion, 

 for which I will quote only a fingle authority. " A ray of light will not pafs out of glafs 

 into air, if the angle of incidence exceeds 40° 11'." Adam's Ledlures on Natural and 

 Experimental Philofophy, London, 1794. ii- 163. The determination of Newton does not 

 fenfibly differ from this: " Totalis reflexio turn incipit, cum angulus incidentiae fit 40° 10'." 

 Opt. II. p. 3. prop. I. Can we affirm that under the angle of 45°, there f>ill pafTes a 

 fufficient number of rays ta render the experiment preceptible ? Or mufl we fuppofe that 

 the ray F M, was not exactly perpendicular to the face AC? I think the latter is the truths 

 that is to fay, that, in the firfl pofition of the prifm, the ray began by being oblique to A C, 

 in the oppofite fituation to that indicated in fig. 2. So that the angle A P F was lefs than 

 F P C. Whence it would refult, that the mofl refrangible rays would fall on the pofterior- 

 furface, B C, under a greater angle of incidence, and, therefore, more favourable to reflec- 

 tion. Under this form the argument of Mr. Brougham acquires new force. 



The optics of Newton, in this place, requires a commentary. The beft will be that 

 which his Leftiores Opticas prefent us — If. Newtoni Opufcul. Laufannce et Geneva, 1744- 

 ii. 217. 223. Here the author replies to our doubts as follows: " Ne qua oriatur furpicio, 

 quod refraftiones in fuperficiebus, AC et A B, ad ingrefTum radiorum in prifma et egref- 

 fum fa£la, poflint aliquid conducere ad efFeiStus hofce producendos, obfervare licet quod 

 efFeftus iidem producuntur, cujufcunque magnitudinis ftatuatur angulus A C B * ; hoc eft, 



quaecunque fit refraclio fuperficiei AC. Imo poffis efficere quod cum colores partim 



refledluntur . . . . et partim trajiciuntur radii perpendiculariter incidant in A G, 



emergantque ex A B, et fie neutra fuperficie refringantur, modo ftatuas angulum A C B, 

 elTe grad. 40 circiter, et iidem tamen efFedus produccntur \." p. 219. The moft accUralfe 



writers 



* It is ABC, in the text ; manifeftly by an error of the prefs. This is, indeed, of no confequencf to 

 my objcft. I do not quote what relates to the equality required between the angles B and C, becaufe it does 

 not aifeft the prefent enquiry. 



-)• Anglke. Left any fufpicion ihould arife, that the refradions at the furfaces AC, ajid AB, at the 



eatranca 



