[Greek: ge], [Greek: eê], [Greek: êi], [Greek: il], and [Greek: l]G, will be the Spaces which the several Colours (red, orange, yellow, green, blue, indigo, violet) take up. [Illustration: FIG. 4.] [Illustration: FIG. 5.] Now these Intervals or Spaces subtending the Differences of the Refractions of the Rays going to the Limits of those Colours, that is, to the Points M, [Greek: a], [Greek: g], [Greek: e], [Greek: ê], [Greek: i], [Greek: l], G, may without any sensible Error be accounted proportional to...