320 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q8. 



XL Some Optical Remarks, chiefly relative to the Reflexibility of the Rays of 

 Light. By Mr. P. Prevost, Profess, of Philos. at Geneva, &c. From the 

 French, p. 311. 



The word reflexibility, says Mr. P., is taken in 2 different senses. 1. Newton 

 means by it, that property of a ray of homogeneal light, by which the ray is re- 

 flected if it fall in a certain angle of incidence, and transmitted if it fall in a less 

 angle : or, more simply, a disposition to be reflected, and not transmitted, at the 

 limit which separates 2 refracting mediums. (Opt. 1. 1, pt. 1, dif. 3). That phi- 

 losopher thinks, that in this sense the reflexibility of the rays is not the same. 

 He establishes by experiments, which he deems conclusive, that the most refran- 

 gible rays are also the most reflexible. So that, if a white beam fall in a certain 

 angle on the obstructing surface, the violet ray will be reflected, while the other 6 

 will be still transmitted and refracted. But by augmenting the angle of inci- 

 dence, we shall obtain successively the reflexion of all the rays, from the violet, 

 which is the most reflexible, to the red, which is the least. — Mr. Brougham 

 however (Philos. Trans., for 17Q6, p. 272), does not think those experiments of 

 Newton conclusive, and on another experiment he establishes the contrary prop, 

 viz. that all the rays have the same disposition to be reflected, while the angle of 

 incidence is the same. 



Mr. Brougham means by reflexibility, a disposition to be reflected near the per- 

 pendicular to a certain degree; or a property of a homogeneal ray, by which its 

 angle of reflexion is to the angle of incidence in a certain ratio, which is not that 

 of equality, except in some cases which he indicates. According to him, this 

 ratio varies for each homogeneous ray. The ratio of equality takes place for the 

 rays at the confines of the blue and green : the ratio of inequality for the others ; 

 and the most refrangible are the least reflexible. So that, for the red ray the 

 angle of reflexion is less than the angle of incidence, but for the violet greater. 

 Whereas Newton affirms, on the contrary, that the angle of reflexion is always 

 equal to the angle of incidence. — Let us examine these opposite sentiments. 



Of the 2 experiments by which Newton establishes the inequality of the reflexi- 

 bility of the rays, it will suffice to notice that which Mr. B. attacks directly. 

 Newton, in exper. Q, makes a white beam fall perpendicularly on the anterior 

 face of a prism: then, turning the prism on its axis, he observed the reflexion 

 which took place at its posterior face. He saw the violet reflected first ; then the 

 other rays, in the order of their refrangibilities, till the red which was reflected last. 

 Hence he concludes, that the violet is reflected at a smaller angle of incidence than 

 the red. In attacking this conclusion, Mr. Brougham says, " That the demonstra- 

 tion involves a logical error, appears pretty evident: when the rays, by refraction 

 through the base of the prism used in the experiment, are separated into its parts, 

 these become divergent, the violet and red emerging at very different angles, 

 and these were also incident on the base at different angles, from the refraction of 



