548 PHILOSOPHICAL TRANSACTIONS. [aNNO I67I. 



Hence, that I might examine the nature and difference of both, I put upon 

 some object, as the point A, the prism of double refracting crystal NPRQTBS, 

 (fig. 9, pi. 13,) and the eye M, being perpendicularly posited over the upper 

 plain of the prism, I noted whether there was any refraction of the point A, for 

 the usual laws of refraction teach that there is none. But the perpendicular 

 ray of the eye was observed to pass not through the moveable but the fixed 

 image; thereby being conformable to the rules of usual refraction, as striking 

 the eye unrefracted, so that the eye, the image, and the object, were seen in 

 the same line. But when in the same situation of the eye, the object A did 

 also exhibit the other image X, at no small distance from the former ; I took 

 notice, that this object A was not seen unrefracted by the means of the image 

 X, though the eye M remained perpendicular over the plain; and that con- 

 sequently this unusual refraction is not subject to the received axiom of diop- 

 trics, which imports, that a ray falling perpendicularly on the superficies of a 

 diaphanous body, is not refracted, but passes unrefracted. 



Next, I so placed the eye in O, that the ray from the object A, arriving to 

 the eye, might be parallel to the lines RT and QB, of the plane RQTB, &c. 

 then it appeared that the rays were trajected from the object A, without re- 

 fraction, through the moveable image Q ; the object A, the moveable image Z, 

 and the eye O, being in the same line; and that the same object A did trans- 

 mit to the eye O, remaining in the same position, yet another species Y, through 

 the refracted ray AYO. Whence it was manifest, that this unusual refraction 

 had for its rule the parallel of the sides of this double refracting crystal, while 

 the usual refraction was directed according to the perpendicular of the super- 

 ficies. 



But since the place of the point, appearing through this diaphanous body, 

 cannot easily be determined, as being only obvious in the uppermost part; we 

 shall add the way whereby we have found its diversity, by drawing on the sub- 

 jacent table, a straight line through that point; the place of which line will be 

 determined by the one eye through this crystal, and by the other eye without 

 the crystal. For, in the same figure, through the object A, let be drawn on the 

 table a straight line BC. The eye being in M, that double line HD and IE 

 will appear, the species being cast on the upper surface : And if you attend well, 

 you will observe one of the images, viz. the fixed HD, to be congruent to the 

 subjacent line BC, while the other, namely, the moveable EI, tends towards R. 

 But if afterw'ards the eye be posited in O, the same object BC, will not only be 

 represented double by the images KF and LG; but also the moveable image 

 GL be congruent to the inferior line BC; while the fixed FK is not so, but 

 tends towards N. 



