728 PHILOSOPHICAL TRANSACTIONS. [aNNO 1672. 



considered as parallel, with respect to the inclination to the axis, since they are 

 both parallel to it. But the refraction through two parallel plane surfaces is 

 accounted none, because by how much a ray is refracted one way by the first 

 surface, by just so much is it refracted the contrary way by the other surface. 

 Therefore since the solar rays, transmitted by a hole through a prism, are not 

 refracted sideways, they proceed in that respect as if no prism at all stood in their 

 way, that is with regard to the lateral divarication; but when the same rays on 

 the superior and inferior parts are refracted, some more, some less, as being 

 unequally inclined, they must needs diverge more, and consequently be extended 

 in an oblong figure. 



But when a calculation is rightly made, as the lateral rays were found by Mr. 

 Newton, of a breadth that subtended an arc of 3l', which answers to the sun's 

 diameter; so there is no doubt but the length of the image, which subtended 

 2° 49', would correspond with the same diameter after the unequal refractions. 

 Thus, supposing the prism at ABC, (fig. 7. pi. 15,) having the angle A of 

 60°; and a ray DE making with the perpendicular EH an angle of 30°; after 

 emerging in the line FG, I find it makes with the perpendicular FI an angle of 

 76° 22'. But taking another ray dE, which makes with the perpendicular EH 

 an angle of 29° 30', I find that, when it emerges by {g, it makes with the per- 

 pendicular f i, an angle of 78° 45'. Hence those two rays DE, dE, which are 

 supposed to proceed from opposite parts of the solar disk, and forming between 

 them an angle of 30', where they emerge by the lines FG, fg, they diverge 

 so as to form between them an angle of 2° 23'. And if two other rays were 

 assumed approaching nearer the perpendicular E H, as suppose one of them 

 forming with it an angle of 29° 30', and the other 29°; these rays, after 

 emerging, would diverge still more, and form a greater angle, even sometimes 

 more than 3°. And besides, this distance between the refracted rays is further 

 increased, on this account, that the two rays D E, dE, meeting in E, begin im- 

 mediately to diverge, and then fall on two distant points of the second surface, 

 viz. in F and f. Therefore, in order to render the calculation just, it is not 

 sufficient barely to subduct the diameter of the hole from the length of the 

 image ; for supposing the hole E to be invisible, or almost nothing, yet there 

 would be formed a great hole as it were in F f, in the second surface of the 

 prism. 



What the author calls the Experimentum Crucis, seems also to agree with the 

 commonly received laws of refraction. For, as was just now shown, the sun's 

 rays, which approaching and converging from an angle of 30', coming from an 

 invisible hole, do afterwards diverge in an angle of two or three degrees. It is 

 not then to be wondered at^ if these rays falling severally on a second prism^ 



