322 PHILOSOPHICAL TRANSACTIONS. [ANNO ] 798. 



stances for reflexion than the latter; and is even reflected while the latter is not. 

 It is therefore right to conclude that it is naturally more reflexible in the Newto- 

 nian sense. 



Thus then the consideration introduced by Mr. B. (and which is very just), 

 causes us to conclude a fortiori in favour of the Newtonian assertion. We might 

 say indeed, that not only at the same incidence, the violet ray is reflected while 

 the red is not, but we might even say that the phenomenon takes place though 

 the incidence of the violet be more unfavourable for reflexion than that of the red. 

 Therefore, finally, the rays differ in reflexibility in Newton's sense; and the most 

 refrangible is the most reflexible. Mr. P. quotes also several other authors, all 

 confirming the same proposition; and then proceeds thus. 



But Mr. B. supports the contrary opinion by an experiment which he thus an- 

 nounces: " I held a prism vertically, and let the spectrum of another prism be re- 

 flected by the base of the former, so that the rays had all the same angle of incidence; 

 then, turning round the vertical prism on its axis, when one sort of rays was trans- 

 mitted or reflected, all were transmitted or reflected."* The complete discussion of 

 this experiment, says Mr. P., would require long details; but suffice it to observe, 

 that the plane of the vertical face, on which the refraction acts, cannot be so adjusted 

 as to produce a same angle of incidence with all the rays of a spectrum at once; and, 

 even supposing the thing possible, and executed in an instant, the rotation of the 

 prism had changed that disposition, in altering unequally that angle, fordifferent rays. 

 We may consequently conceive a multitude of different results; among others, we 

 can conceive that the angles of incidence of divers rays may be such, that Mr. B.'s 

 observation will accord with Newton's sentiment, on their unequal reflexibility. 

 But as Mr. B. does not enter into this detail, and gives only one result, it is to be 

 presumed that he did not repeat or vary the experiment. He appears even riot to 

 give much importance to it, by the rapid manner in which he announces it. I 

 think then, that it cannot as yet invalidate Newton's conclusions; and that it is 

 still right to assert, in his sense, that the most refrangible rays are also the most 

 reflexible. 



As to the 2d question, or whether homogeneous rays differ in reflexibility in 

 Mr. B.'s sense ? In other words, with a same angle of incidence, whether the red 

 ray forms an angle of reflexion less, and the violet an angle of reflexion greater, 

 than the angle of incidence ? 



The fundamental experiment whence Mr. B. deduces this unequal reflexibility, in 

 the sense of his definition, is this. A smooth and brilliant cylinder, of a very 

 small diameter, or a metallic thread, having its convexity presented to a white ray, 

 reflected a coloured spectrum ; and, every thing being measured or calculated pro- 

 perly, he perceived that it was only the rays in the confines between the blue and 



* The same experiment, tried by Newton, gave him exactly the contrary result. Lect. Opt. 

 OpuscuL torn. 2, p. 220. — Orig. 



