462 POPULAR SCIENCE MONTHLY. 



trary ways, and so by the latter returned into that course from which 

 the former had diverted it. For, by this means, I thought the regular 

 effects of the first prism would be destroyed by the second, but the 

 irregular ones more augmented, by the multiplicity of refractions. 

 The event was, that the light, which by the first prism was diffused 

 into an oblong form, was by the second reduced into an orbicular one, 

 with as much regularity as when it did not at all pass through them. 

 So that, whatever was the cause of that length, it was not any con- 

 tingent irregularity. 



I then proceeded to examine more critically, what might be effected 

 by the difference of the incidence of rays coming from divers parts 

 of the sun; and to that end measured the several lines and angles, 

 belonging to the image. Its distance from the hole or prism was 22 

 feet; its utmost length lSy^ inches; its breadth 2%; the diameter of 

 the hole 14 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% 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 subtended, was about 31', 

 answerable to the sun's diameter; but the angle which its length sub- 

 tended, 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 re- 

 fractions of two rays flowing from opposite parts of the sun's discus, 

 so as to differ 31' in their obliquity of incidence, and found that 

 the emergent rays should have comprehended an angle of about 31', as 

 they did, before they were incident. But because this computation 

 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 31', 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 con- 

 sequently by that variation of incidence, the quantity of refraction 

 was not sensibly varied. By this experiment therefore, as well as 



