VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 321 



the side at which they entered; when therefore the prism is moved round on its 

 axis, as described in the proposition, the base is nearest the violet, from the posi- 

 tion of the rays by refraction, and meets it first ; so that the violet being reflected 

 as soon as it meets the base, it is reflected before any of the other rays, not from 

 a different disposition to be so, but merely from its different refrangibility." Thus 

 Mr. B. thinks that the reflexion of the violet ray precedes the red one, only be- 

 cause the refraction at the anterior surface forces the violet ray to attain the poste- 

 rior surface sooner than it causes the red. 



But, says Mr. P., it seems that the effect is here in the inverse sense of the 

 cause. It is impossible that the author could wish to say that the eye can seize the 

 interval of time between the arrival of the violet and red ray at the posterior face 

 of the prism. Now, of the 2 rays, that which describes the shortest route, falls 

 nearest the perpendicular let fall from the point of departure: and hence alone we 

 may conclude that it falls in a less angle of incidence. From the premises it fol- 

 lows, that it is the red ray that must be first reflected, and not the violet. Now, 

 let us first consider the position of the prism at the first moment, and such as the 

 figure given by Newton in his Optics. The white beam fm, fig. 3, pi. 5, is per- 

 pendicular on ac: in this case it is not refracted at its immergence, but pursues the 

 right line fm. At this point Newton represents the violet ray only, mn, re- 

 flected, while all the others, as mh, mi, are transmitted and refracted: at least, 

 the violet alone is reflected entire. 



Yet it is certain that he has failed, for obtaining this phenomenon, to endeavour, 

 by turning the prism, to give it the degree of inclination that can make the expe- 

 riment succeed: and Mr. B. is right in observing, that from thence the perpendi- 

 cularism of the ray on the anterior face, ac, has ceased; that in consequence it 

 has suffered a refraction, and that the several homogeneal rays have not followed a 

 rectilineal route, as fm, and have not met the posterior face bc in equal angles. 

 Let now a'bV, fig. 4, be the new position of the prism, taken by the rotation on 

 its axis: then the ray fp will fall obliquely on a'c', at the point p; so that the 

 perp. po will be towards the side a', or within the angle a'pf. This is what re- 

 sults, 1st, from the end proposed by Newton, viz. of augmenting the angle of 

 incidence on the posterior face; which angle would be too little to produce re- 

 flexion. 2. From Newton's precise expressions, when he says that the prism abc 

 is turned on its axis according to the order of the letters, a, b, c. The ray fp 

 then, in fig. 4, will be refracted towards the perpendicular op : but the most re- 

 frangible ray, the violet, will approach it nearest; and the least refrangible (the red) 

 will approach it the least; as in the two lines pv, pr: so that the violet ray makes 

 with the posterior face bV an angle, pvc', greater than the angle prc formed by 

 the red: but the angles of incidence, at the points v, r, are the complements of 

 the angles pvc', prc', respectively. It is certain then, that in virtue of the re- 

 fraction at the anterior face, the violet ray meets the posterior face in a less angle 

 of incidence than the red; consequently the former is in more favourable circum- 



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