_Wherever the Rays which come from all the Points of any Object meet
again in so many Points after they have been made to converge by
Reflection or Refraction, there they will make a Picture of the Object
upon any white Body on which they fall._
So if PR [in _Fig._ 3.] represent any Object without Doors, and AB be a
Lens placed at a hole in the Window-shut of a dark Chamber, whereby the
Rays that come from any Point Q of that Object are made to converge and
meet again in the Point _q_; and if a Sheet of white Paper be held at
_q_ for the Light there to fall upon it, the Picture of that Object PR
will appear upon the Paper in its proper shape and Colours. For as the
Light which comes from the Point Q goes to the Point _q_, so the Light
which comes from other Points P and R of the Object, will go to so many
other correspondent Points _p_ and _r_ (as is manifest by the sixth
Axiom;) so that every Point of the Object shall illuminate a
correspondent Point of the Picture, and thereby make a Picture like the
Object in Shape and Colour, this only excepted, that the Picture shall
be inverted. And this is the Reason of that vulgar Experiment of casting
the Species of Objects from abroad upon a Wall or Sheet of white Paper
in a dark Room.
In like manner, when a Man views any Object PQR, [in _Fig._ 8.] the
Light which comes from the several Points of the Object is so refracted
by the transparent skins and humours of the Eye, (that is, by the
outward coat EFG, called the _Tunica Cornea_, and by the crystalline
humour AB which is beyond the Pupil _mk_) as to converge and meet again
in so many Points in the bottom of the Eye, and there to paint the
Picture of the Object upon that skin (called the _Tunica Retina_) with
which the bottom of the Eye is covered. For Anatomists, when they have
taken off from the bottom of the Eye that outward and most thick Coat
called the _Dura Mater_, can then see through the thinner Coats, the
Pictures of Objects lively painted thereon. And these Pictures,
propagated by Motion along the Fibres of the Optick Nerves into the
Brain, are the cause of Vision. For accordingly as these Pictures are
perfect or imperfect, the Object is seen perfectly or imperfectly. If
the Eye be tinged with any colour (as in the Disease of the _Jaundice_)
so as to tinge the Pictures in the bottom of the Eye with that Colour,
then all Objects appear tinged with the same Colour. If the Humours of
the Eye by old Age decay, so as by shrinking to make the _Cornea_ and
Coat of the _Crystalline Humour_ grow flatter than before, the Light
will not be refracted enough, and for want of a sufficient Refraction
will not converge to the bottom of the Eye but to some place beyond it,
and by consequence paint in the bottom of the Eye a confused Picture,
and according to the Indistinctness of this Picture the Object will
appear confused. This is the reason of the decay of sight in old Men,
and shews why their Sight is mended by Spectacles. For those Convex
glasses supply the defect of plumpness in the Eye, and by increasing the
Refraction make the Rays converge sooner, so as to convene distinctly at
the bottom of the Eye if the Glass have a due degree of convexity. And
the contrary happens in short-sighted Men whose Eyes are too plump. For
the Refraction being now too great, the Rays converge and convene in the
Eyes before they come at the bottom; and therefore the Picture made in
the bottom and the Vision caused thereby will not be distinct, unless
the Object be brought so near the Eye as that the place where the
converging Rays convene may be removed to the bottom, or that the
plumpness of the Eye be taken off and the Refractions diminished by a
Concave-glass of a due degree of Concavity, or lastly that by Age the
Eye grow flatter till it come to a due Figure: For short-sighted Men see
remote Objects best in Old Age, and therefore they are accounted to have
the most lasting Eyes.
[Illustration: FIG. 8.]