12 On the UNEQUAL 



fraction of the mean refrangible ray, which obtains in that me- 

 dium, I took a direct method, fimilar in principle to that em- 

 ployed by Sir Isaac Newton, and defcribed by him in the 

 feventh propofition of the firft book of his Optics, and likewife 

 in his Optical Lectures, p. 54. ; but which I may venture to fay 

 will be found much eajier, and perfectly accurate. 



Instead of caufing the rays to pafs through the fights of a 

 large and accurate quadrant, at the diftance of ten or twelve 

 feet, as directed by Sir Isaac Newton, I employed a Had- 

 ley's quadrant, in the following manner : 



Fig. 1. — I reprefents the index-glafs and H the horizon- 

 glafs of a Hadley's quadrant. S I reprefents a folar ray, in- 

 cident on the index-glafs, thence reflected to the horizon-glafs 

 H, and from it to the eye at E. The line s g reprefents another 

 folar ray, incident on the«prifm P, and through it refracted to 

 the eye at E. When the prifm is turned flowly round its axis>, 

 till the fpectrum G appears at its greateft height, this is its pro- 

 per pofition. The angle formed by the direct and refracted 

 ray is then the lead pomble, and the angles of incidence and 

 emergence are equal. Let the prifm be fecured in this pofition. 

 A flight infpection of the figure will fhew, that when the re- 

 flected and refracted images of the fun are made to coincide, 

 the angle marked by the index of the quadrant, is the fame 

 which the incident ray s g forms with the refracted ray P E 

 produced. For S Z H is the angular diftance of the fun and 

 his doubly reflected image, marked by the index ; and the an- 

 gle sgG, which the ray incident on the prifm forms with the 

 refracted ray produced, is equal to it; s g and S t I being paral- 

 lel, and P Z and H Z being coincident. 



The manner in which the ratio of the fines of the. angles of 

 incidence and refraction may be computed from the above an- 

 gle, and the refracting angle of the prifm being given, is fully 

 explained in the celebrated works which have juft been quoted. 



It 



