On the unequal ReJtexihUlt'j of Light. 227 



all the rays of the fpe£trum at once under the fame angle ; and even fuppofing this to have 

 been poffible, and done for an inftant, yet the rotation of the prifm v/ould have changed this 

 difpofition, by altering this angle with regard to the feveral rays. We may, confequently, 

 imagine a variety of different rcfults ; and, among others, we may conceive the angles of incidence 

 of the feveral rays to be fuch, that the obfervation of Mr. Brougham may be reconciled with the 

 opinion of Newton, on their unequal reflexibility. But fince Mr. Brougham does not enter 

 into this detail, and gives only a fingle refult, it is to be prefurtied, that he has not repeated, 

 or at leaft varied, this experiment. This philofopher even appears, by his rapid enunciation, 

 to have confidered it of no very great importance. 



1 think, therefore, that it cannot at prefent weaken the conclufions of Newton ; and that 

 we are ftill juftified in affirming, in the fenfe of that philofopher, that the moft refrangible 

 rays are alfo the moft reflexible. 



Seifion 4. Second quest ion. Do the rays of light differ in reflexibility, in the 

 Broughamian fenfe ? In other words, does the red ray at the fame angle of incidence form a 

 lefs angle of reflection, and the violet a greater, than the angle of incidence ? 



Section 5. The fundamental experiment from which Mr. Brougham deduces this unequal 

 reflexibility in the fenfe of his definition, is this : 



A bright poliftied cylinder, of very fmall diameter (a metallic fibre), having its convex 

 furface prefented to a white ray, reflefted a coloured fpedtrum ; and after proper admeafure- 

 ment and computation, it appeared that the rays in the confine of the blue and green, were 

 alone refleiSted, under an angle equal to that of the incidence. The red rays were refledled 

 under a lefs angle, and the violet under a greater. 



The queft:ion, therefore, is reduced to determine, whether this experiment be conclufive in 

 favour of Mr. Brougham. 



Se£lion 6. To afcertain this, it is of importance to advert to a principle laid down by New- 

 ton, and admitted by Brougham, p. 250. (or Philof, Journal, i, 561), namely, that the force, 

 whatever it may be, that produces refle6lion, a6ts in the diredlion of a line, perpendicular to 

 the refleding furface. 



Seiiion 7. From this principle it follows, that the reflection operated by a plane furface, 

 muft be made according to the law hitherto admitted by all opticians (Newton's Princip. 1. i. 

 p. 96). And this is true, whatever maybe the intenfity of the repulfive force, and the incli- 

 nation of the incident ray ; provided the ray be reaUy incident, and do not move parallel to 

 the repulfive furface. 



Se£itbn 8. This confequence, and the whole ofjthe Newtonian demonftration, fuppofes that 

 the furface a6ts on the ray during its whole courfe through the fphere of its activity ; and 

 equally at equal diftances. Mr. Brougham alleges nothing againft this hypothefis, but even 

 feems to admit it exprefsly^p. 269, or Ph. Journal, i. 593) ; and indeed how can it be denied.^ 



Seifion 9. Therefore, from a principle which is not contefted, it appears that refleflion 

 cannot decompofe white light, when it is made totally from a plane furface. 



This is perfedly conformable to the obfervation of Mr. Brougham, that there is no mean 



of 



