724 PHILOSOPHICAL TRANSACTIONS. [anNO 1763. 



converge in the water, through which a plane may be drawn which shall send them 

 out into the air, in a colourless pencil o s. 



Remark 1. The 8th experiment in Sir Isaac Newton's Optics, book 1, part 2, 

 seems to have been made under the conditions which are limited by the foregoing 

 problem ; though he does not specify these conditions. For it is to be presumed 

 he did not combine his prism and water at random, but adjusted them so as to 

 produce the expected effect. It is observed also, that he does not give us a de- 

 scription of his experiment so particular as, in most instances, he was wont to do. 

 He thought perhaps that the consequences he deduces from it might sufficiently 

 explain his meaning ; especially as he had, in the foregoing propositions, fully 

 established the principles of his theory. However this be, several persons of skill 

 and address in optical matters, have produced experiments in contradiction to that 

 of Sir Isaac, and have affixed meanings to his conclusions which he never could 

 intend, without being grossly inconsistent with himself: an imputation from 

 which common candor and decency ought to have protected so great a name.* 



For instance, when he says that " light as often as by contrary refractions it is 

 so corrected that it emergeth in lines parallel to those in which it was incident, 

 continues ever after to be white;" can this assertion possibly bear the meaning 

 they would obtrude upon us ? Had Sir Isaac so entirely forgot his own doctrine 

 as not to know, that if the glass prism p nw, in the last scheme, be any where 

 above vv, terminated by a plane to which the pencil s o is perpendicular, the rays 

 vv, Er, &c. though emerging parallel to s o, will exhibit their several colours ? 

 The sense therefore which the experiments affix to Sir Isaac Newton's words being 

 so absurd, had not they done better to look out for one that was consistent with 

 his theory ? and such a one they would have found by only drawing a figure like 

 the foregoing ; where the rays of the pencil, reunited in os as well as when sepa- 

 rated within the glass prism, are parallel to each other and to the incident pencil. 

 But if the water be terminated by a plane different from a c, passing through the 

 point 0, and making the rays (no longer parallel to s o) to diverge, then the light 

 will by degrees, in passing on from o, become coloured ; which is Sir Isaac's 

 other position. To this meaning his own words ought to have led the objectors. 

 It was light, not separate rays, which emerged in his experiment : and which 

 (being parallel to the incident light) continued to be colourless. He adds farther, 

 " the permanent whiteness argues, that in like incidence of the rays, there is no 

 separation of the emerging rays ;" as much as to say, that in his experiment (as 

 in our 6th fig) the pencil, in passing or repassing, is supposed to meet with sur- 

 faces of equal refractive powers, similarly situated. 



• The reader ought to be told, that it is not here intended to detract from the merit of the late 

 Mr. Dollond's improvement of refracting telescopes ; but only to correct a mistake of his concemiog 

 that difference of dispersion of rays, which he has so happily applied to use. — Orig. 



