VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 323 



green that were reflected in an angle equal to the angle of incidence: the red were 

 reflected at a less angle, the violet at a greater. Now the question reduces to this, 

 to know if that experiment be conclusive in favour of Mr. B.s statement. 



To ascertain this, it is material to recollect a principle laid down by Newton, 

 and admitted by Mr. B., that the force producing reflexion, acts perpendicularly 

 to the reflecting surface. From this principle it follows, that the reflexion pro- 

 duced by a plane surface, ought to act by a law hitherto admitted by all opticians. 

 And this is true, whatever be the intensity of the repulsive force, and also, what- 

 ever be the velocity and the inclination of the incident ray: provided the ray be 

 really incident, and move not parallel to the repulsing surface. Thus, according 

 to a principle which is not contested, it appears that the reflexion cannot decom- 

 pose the white light, when this is wholly reflected by a plane surface. This is 

 perfectly conformable to what Mr. B. observes, that by no means can we succeed 

 to effect this decomposition, in using plane surfaces, nor curve surfaces with a ray 

 extremely small, and as it were evanescent. We may in fact, conceive that an 

 element of the curve surface of a large ray, is a real plane, for a particle of light. 

 The author indeed explains this phenomenon in another manner; but the fact, 

 independent of all explanation, is not less certain and acknowledged. After some 

 further remarks, Mr. P. concludes, from all that has been said, that homogeneous 

 rays are not unequally reflexible in Mr. B.'s sense; in other words, that the law of 

 reflexion admitted by Newton, is the true law of nature. 



It follows from the preceding discussion that the violet rays reflect soonest, and 

 the red most forcibly. Even when these 2 effects may have taken place in like 

 circumstances, they would not be perhaps incongruous. We might conceive that 

 the sphere of activity extends a little farther for the violet than for the red, but that it 

 acts on these with more intensity. But it is essential to remark that these 2 effects 

 take place in very different, and even opposite circumstances: and this indicates an 

 important exception to Newton's assertion, on the unequal reflexibility of divers 

 homogeneal rays. In the experiments by which this philosopher establishes it (exper. 

 9 and 10), reflexion acts in the denser medium, and therefore, by attraction. On the 

 contrary, in Mr. B.'s experiments reflexion acts in the rarer medium, that is by 

 repulsion. 



Thus, on the one part, we see that the most refrangible rays, or the most at- 

 tracted in the act of transmission, are also the most attracted in the act of re- 

 flexion ; and, on the other part, we see that the least refrangible rays, or the least 

 attracted in transmission, are the most repulsed, or the least attracted, in the act 

 of reflexion. This appears to make an exception to the Newtonian law of unequal 

 reflexibility; since this unequal reflexibility is proved by Newton only for the case 

 where the ray moves in the densest medium. I do not recoll e ct that Newton, or 

 any other optician, till Mr. B., has treated the other case. The experiments of 

 this last philosopher appear to me to indicate, at least indirectly, the Newtonian 

 unequal reflexibility, for the case omitted by Newton, viz. where the ray moves in 

 the rarest medium : and hence there follows, I think some disposition to believe, 



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