224 Oti the Ulttqual ReJIexibility of Light. 



in the optics of Newton, The white ray F M, plate xvi. hg. i, is perpendicular to A C ; 

 and, in this cafe, it is not refrafted, but defcribes a right line. At this point, Newton re- 

 prefents the violet ray M N,as alone refleded; while all the others, fuch as JVI H, M I, 

 are tranfaiitted and refracted (at leaft the violet alone is totally refle£lcd). 



It is, neverthelefs, certain, that in order to obtain this phenomenon, it was requifite to 

 give the proper degree of inclination, by turning the prifm, in order that the experiment 

 might fucceed : and Mr. Brougham has very properly obferved, that the perpendicularity of 

 the ray on the anterior face A C muft then ceafe ; confequently, there mufl: have been 

 refraftion, and the feveral homogeneous rays could not have followed a right-lined courfe, fuch 

 as F M, nor have fallen on the pofterior face B C, under equal angles. 



Let A'B'C, fig. 2, be the new pofition of the prifm, which it has taken by virtue of its 

 notation on its axis ; the ray F P will then fall obliquely upon A C, at the point P, fo that 

 the perpendicular P O may be on the fide A, and, confequently, comprized in the angle 

 APF. The refult will be, i. What Newton propofed, namely, to increafe the angle of 

 incidence on the pofterior furface ; which angle f formed at the point M, in fig. i, which 

 reprefents the firft pofition of the prifm) was before too fmall to produce refledion ; and, 

 2. The precife expreflions of Newton affirm, that the prifm was turned on its axis, accord- 

 ing to the direction indicated by the order of the letters A B C, in his figure, which, as far 

 SIS relates to my objedt, is the fame as my figure i. 



The ray F P, fig. 2, will, therefore, be refra£ted, in approaching the perpendicular O P, 

 but the moft refrangible ray (the violet) will approach the neareft, and the leaft refrangible 

 will be the fartheft ofF. So that the courfes of the rays refpedively, will be well rcprefented 

 by the lines P V, PR. The violet ray will, therefore, make, with the pofterior face B C, 

 an angle P V C, greater than the angle P R C, formed by the red. Now the angles of in- 

 cidence at the points V, R, are the complements of the angles P V C, P R C, refpec- 

 tively. 



It is, therefore, certain, that by virtue of the refradion, which is produced at the anterior 

 furface, the violet ray meets the pofterior furface under a lefs angle of incidence than the 

 red ; and, confequently, the former is under lefs favourable circumftances for refledlion thaa 

 the latter ; yet the former is refledled before the latter. We are, therefore, right in conclud- 

 ing, that it is in its own nature more reflexible, in the Newtonian fenfe of the word *. 



This very proper confideration, which was introduced by Mr. Brougham, affords, 

 therefore, a ftill ftronger conclufion in favour of the Newtonian affertion. We may fay, 

 that the violet is not only refleiled at the fame incidence at which the red is tranfmitted ; but, 

 likewife, that this phenomenon takes place, though the angle of incidence of the violet be 

 more unfavourable to refleftion than that of the red. 



• This reafoning is equally applicable to the tenth experiment of Ivewton, in which he ufcs two prifms 

 joined together, to form one paiallelopipedon. In both the experiments (yth and loth), another acceffory 

 prifm was ufed, to render the cflfeft more fenfible, by difperfing the reflefting rays. It was unnecelTary to 

 attend to this, 



In, 



