So then the Meaning of this Axiom is, that if Rays fall upon any Plane
or Spherical Surface or Lens, and before their Incidence flow from or
towards any Point Q, they shall after Reflexion or Refraction flow from
or towards the Point _q_ found by the foregoing Rules. And if the
incident Rays flow from or towards several points Q, the reflected or
refracted Rays shall flow from or towards so many other Points _q_
found by the same Rules. Whether the reflected and refracted Rays flow
from or towards the Point _q_ is easily known by the situation of that
Point. For if that Point be on the same side of the reflecting or
refracting Surface or Lens with the Point Q, and the incident Rays flow
from the Point Q, the reflected flow towards the Point _q_ and the
refracted from it; and if the incident Rays flow towards Q, the
reflected flow from _q_, and the refracted towards it. And the contrary
happens when _q_ is on the other side of the Surface.
AX. VII.
_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
11