VOL. XXXV.] I'HILOSOPHICAL TRANSACTIONS. 303 



zetti's mistakes; therefore the Dr. only mentions one more experiment from 

 Sir Isaac Newton, which he repeated on account of what is said in Rizzetti's 

 Preface, p. l6, viz. that if, according to Sir Isaac, rays were differently re- 

 flexible, colours must be produced by reflection from a plane surface; but this, 

 says our author, is contrary to experience. Now this his assertion is dis- 

 proved by 



Exper. 9.— As this experiment was made exactly in Sir Isaac Newton's 

 manner, and with the same success, the Dr. refers to his own account of it, 

 in Book 1, Part 1, Exper. ](), as illustrated by fig. 20. 



If this need any farther explanation, let us suppose cab, the section of the 

 prism in fig. 20, transferred to fig. 21, at acb. If ro be a red ray, inclined 

 to a perpendicular to ab, in an angle of more than 41 or 42 degrees, it will, 

 at its emersion under the surface ab, be turned into the curve onmi, and so go 

 up again to the eye at e; but another red ray coming in the direction rn, 

 making an angle with the perpendicular sufficiently less, will after its emersion 

 at n, be only bent so much as to be turned out of the way, and refracted to q, 

 in an angle of refraction agreeable to the refrangibility of red light. But vm 

 a violet ray, with the same inclination as the last red one rn, shall not be re- 

 fracted, but turned up in the curve mip, and so go to the eye at e. Another 

 violet ray vm, making an angle something less with the perpendicular, will 

 pass through the glass, and be refracted in the line ms. On this account, all 

 that part of the base of the prism, of which ab is the section, between a 

 and p, will be dark or faint ; all that part between p and n be tinged with a 

 bluish colour; and all between o and b, of a bright white. 



Postscript. — The bending of rays of light, just as they come to be reflected 

 or refracted, may be easily understood by such as are well acquainted with those 

 properties of light, which Sir Isaac Newton calls their fits of easy reflection, 

 and fits of easy transmission; without any hypothesis, but by consequences 

 fairly drawn from experiments and observations. But as Signior Rizzetti does 

 not seem to have the least notion of those properties of light, and the nice 

 observations on which they are founded; and several other persons have not 

 time to read those parts of the optics, with sufficient application, to show hovw 

 the same power of the surface of a dense medium may both attract and repel 



under different circumstances the Dr. contents himself here with giving 



the hypothesis, which Sir Isaac does before he comes to that part of his book 

 where he demonstrates the fits abovementioned. 



If GG, fig. 22, be the surface of a dense medium gddg, on which a tremor 

 is caused by the warmth communicated to it by the rays of light, so as to give 



