A HISTORY OF SCIENCE 



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 refrac- 

 tions on both sides of 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, sub- 

 ducting 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, passing through the centre of the said 

 hole, which that breadth subtended, was about 31', 

 answerable to the sun's diameter; but the angle which 

 its length subtended was more than five such diame- 

 ters, namely 2 49'. 



" Having made these observations, I first computed 

 from them the refractive power of the 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 31' in their obliquity of incidence, 

 and found that the emergent rays should have com- 

 prehended an angle of 31', as they did, before they were 

 incident. 



" But because this computation was founded on the 

 hypothesis of the proportionality of the sines of in- 

 cidence 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 make my 

 prism. And having placed it at my window, as before, 



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