680 PHILOSOPHICAL TRANSACTIONS. [aNNO 1672. 



distance from the hole or prism was 22 feet ; its utmost length 13^ inches ; its 

 breadth 2-^ ; the diameter of the hole 4- of an inch ; the angle, which the rays, 

 tending towards the middle of the image, made with those lines in which they 

 would have proceeded without refraction, was 44° 56' ; and the vertical angle 

 of the prism, 63° 12'. Also the refractions on both sides the prism, that is, 

 of the incident and emergent rays, were as near as I could make them equal, 

 and consequently about 54° 4'. And the rays fell perpendicularly upon the 

 wall. Now subducting the diameter of the hole from the length and breadth 

 of the image, there remains 13 inches the length, and 2-3- the breadth, 

 comprehended by those rays, which passed through the centre of the said 

 hole, and consequently the angle of the hole, which that breadth subtend- 

 ed, was about 3l', answerable to the sun's diameter; but the angle which its 

 length subtended, was more then five such diameters, namely 2° 49'. 



Having made these observations, I first computed from them the refractive 

 power of that glass, and found it measured by the ratio of the sines, 20 to 31. 

 And then, by that ratio, I computed the refractions of two rays flowing from 

 opposite parts of the sun's discus, so as to differ 3l' in their obliquity of inci- 

 dence, and found that the emergent rays should have comprehended an angle 

 of about 31', as they did, before they were incident. But because this com- 

 putation was founded on the hypothesis of the proportionality of the sines of 

 incidence and refraction, which though, by my own experience, I could not 

 imagine to be so erroneous as to make that angle but 3l', which in reality was 

 2® 49' ; yet my curiosity caused me again to take my prism. And having 

 placed it at my window, as before, I observed, that by turning it a little about 

 its axis to and fro, so as to vary its obliquity to the light, more than an angle 

 of 4 or 5 degrees, the colours were not thereby sensibly translated from their 

 place on the wall, and consequently by that variation of incidence, the quan- 

 tity of refraction was not sensibly varied. By this experiment therefore, as 

 well as by the former computation, it was evident, that the difference of the' 

 incidence of rays, flowing from divers parts of the sun, could not make them, 

 after a decussation, diverge at a sensibly greater angle, than that at whicli they 

 before converged ; which being at most but about 31 or 32 minutes, there 

 still remained some other cause to be found out, from whence it could be 2^ 49'. 



Then I began to suspect whether the rays, after their trajection through the 

 prism, did not move in curve lines, and according to their more or less curvity 

 tend to divers parts of the wall. And it increased my suspicion, when I re- 

 membered that I had often seen a tennis ball, struck with an oblique racket, 

 describe such a curve line. For, a circular as well as a progressive motion be- 

 ing communicated to it by that stroke, its parts on that side, where the mo- 



