If the Object A [in FIG. 9.] be seen by Reflexion of a Looking-glass _mn_, it shall appear, not in its proper place A, but behind the Glass at _a_, from whence any Rays AB, AC, AD, which flow from one and the same Point of the Object, do after their Reflexion made in the Points B, C, D, diverge in going from the Glass to E, F, G, where they are incident on the Spectator's Eyes. For these Rays do make the same Picture in the bottom of the Eyes as if they had come from the Object really placed at _a_ without the Interposition of the Looking-glass; and all Vision is made according to the place and shape of that Picture. In like manner the Object D [in FIG. 2.] seen through a Prism, appears not in its proper place D, but is thence translated to some other place _d_ situated in the last refracted Ray FG drawn backward from F to _d_. [Illustration: FIG. 10.] And so the Object Q [in FIG. 10.] seen through the Lens AB, appears at the place _q_ from whence the Rays diverge in passing from the Lens to the Eye. Now it is to be noted, that the Image of the Object at _q_ is so much bigger or lesser than the Object it self at Q, as the distance of the Image at _q_ from the Lens AB is bigger or less than the distance of the Object at Q from the same Lens. And if the Object be seen through two or more such Convex or Concave-glasses, every Glass shall make a new Image, and the Object shall appear in the place of the bigness of the last Image. Which consideration unfolds the Theory of Microscopes and Telescopes. For that Theory consists in almost nothing else than the describing such Glasses as shall make the last Image of any Object as distinct and large and luminous as it can conveniently be made. I have now given in Axioms and their Explications the sum of what hath hitherto been treated of in Opticks. For what hath been generally agreed on I content my self to assume under the notion of Principles, in order to what I have farther to write. And this may suffice for an Introduction to Readers of quick Wit and good Understanding not yet versed in Opticks: Although those who are already acquainted with this Science, and have handled Glasses, will more readily apprehend what followeth. FOOTNOTES: [A] In our Author's _Lectiones Opticae_, Part I. Sect. IV. Prop 29, 30, there is an elegant Method of determining these _Foci_; not only in spherical Surfaces, but likewise in any other curved Figure whatever: And in Prop. 32, 33, the same thing is done for any Ray lying out of the Axis. [B] _Ibid._ Prop. 34. _PROPOSITIONS._ _PROP._ I. THEOR. I. _Lights which differ in Colour, differ also in Degrees of Refrangibility._ The PROOF by Experiments. _Exper._ 1.