On the unequal RcJlexibiHty of Light. 223 



of equality takes place in thofe rays, which occupy the confines of the ))lue and green : that 

 of inequality takes place with regard to the others, and the moft refrangible one the leaft 

 rcflexible. So that with regard to the red ray, the angle of refradion is lefs, and in the 

 violet greater, than the angle of incidence. 



It is known that Newton, on the contrary, affirms that the angle of reflexion is, in all 

 cafes, equal to the angle of incidence. 

 Let us examine thefe oppofite opinions. 



SeSiion 2. The fir Jl qucjilon. Do the homogeneous rays of light differ In reflexibllity in the 

 Newtonian fenfe of the term ? Or, more dire6lly ; does it happen, in faft, that the violet ray 

 will be reflected under the fame angle of incidence at which the red ray is tranfmitted, all 

 other circumftances being precifely the fame ? 



Of the two experiments by which Newton eftabliflies the unequal reflexlbility of the rays, 

 it will be fufficient to mention that which Mr. Brougham attacks direftly. 



Newton caufed a white ray to fall perpendicularly upon the anterior furface of a prifm ; 

 after which, turning the prifm on its axis, he obferved the reflexion which took place, froni 

 its poflerior furface. He faw that the violet ray was firlt reflefted ; and afterwards, the other 

 rays in the order of their refrangibilities, the red ray being reflefted the laft. Whence he 

 concludes, that the violet ray is reflesSted under a lefs angle of incidence than the red. Expe- 

 riment 9. 



This is the conclufion which Mr. Brougham attacks ; and to avoid altering his thought, I 

 fhall here tranfcribe his exprefSons. " That the demonftration involves a logical error, 

 appears pretty evident. When the rays, by refradlion through the bafe of the prifm ufed in 

 the experiment, are feparated into their parts, thefe become divergent, the violet and red 

 emerging at very different angles ; and thefe were alfo incident on the bafe at different ano^les, 

 from the refraiSfion of the fide at which they entered : when, therefore, the prifm is moved 

 round on its axis, as defcribed in the propofition, the bafe is nearefl the violet, from the 

 pofition of the rays by refradion, and meets it firfl: : fo that the violet being reflefted as foon 

 as it meets the bafe, it is reflected before any of the other rays, not from a different difpofi- 

 tion to be fo, but merely from its different refrangibility." 



So that Mr. Brougham thinks that the refledion of the violet ray does not precede that of 

 the red ray ; but becaufe the refradion which takes place at the anterior furface, forces the 

 violet ray to arrive at the pofterior furface fooner than the red ray. 



But it feems that the effedl is here the inverfe of its. caufe. Let us firfl exclude a falfe fenfe 

 of the words. It is impoffible that the author can mean to fay that the eye is capable of 

 appreciating the Interval of time, between the arrival of the violet and the red ray at the 

 pofterior face of the prifm. However, that ray which defcribes the fhortefl courfe, falls 

 neareft the perpendicular let fall from the point of departure; and from this circumftance 

 alone, we can conclude that its angle of incidence is fmaller. Whence it follows, that the red 

 ray ought to be refleded the firft, and not the violet. 



Let us, in fadl, confider the pofition of the prifm at the firft moment, as it is reprefentcd 



G g 2 in 



