_An Object seen by Reflexion or Refraction, appears in that place from
whence the Rays after their last Reflexion or Refraction diverge in
falling on the Spectator's Eye._
[Illustration: FIG. 9.]
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.