364 Sir Isaac Newton’s Book III.

that key, E G the length of the lefier third, E H of thefourth, E I of the fifth, E K of the greater fixth, E L ofthe feventh, and E C of the octave above that key ; that is ifthe lines E D, E F, E G, E H, E I, E K, E L, and E C bear the fameproportion as the numbers, 1, f, £, f, ~, i, ' r > refpedivelythen jfhall B D, BF, be the two limits of the fines of refradtion ofthe violet-making rays, that is the violet-making rays fhallnot all of them have precifely the lame fine of refradtion ,but none of them fhall have a greater fine than B D, nora lefs than B F, though there are violet-making rays whichanfwer to any fine of refradtion that can be taken be-tween thefe two. In the fame manner B F and B Gare the limits of the fines of refradtion of the indigo-ma-king rap ; B G, B H are the limits belonging to the blue-making rays; B H, B I the limits pertaining to the green-ma-king rays, B I, B K the limits for the yellow-making rays;BK, BL the limits for the orange-making rays; and laftly,B L and B C the extreme limits of the fines of refradtion.belonging to the red-making rays. Thefe are the propor-tions by which the heterogeneous rays of light are feparateeffrom each other in refradtion.

8. When light pafies out of glals into air, our authorfound A to B C as yo to 77, and the fame A to B D as yoto 78. And when it goes out of any other refradting lub-ftance into air, the excefs of the fine of refradtion of anyone fpecies of rays above its fine of incidence bears a con-fiant proportion, which holds the fame in each fpecies, todie excels of the fine of refradtion of the lame fort of rays

above