ected most copiously, or by
inclining the Rays begin soonest to be totally reflected, are most
reflexible.

DEFIN. IV.

_The Angle of Incidence is that Angle, which the Line described by the
incident Ray contains with the Perpendicular to the reflecting or
refracting Surface at the Point of Incidence._

DEFIN. V.

_The Angle of Reflexion or Refraction, is the Angle which the line
described by the reflected or refracted Ray containeth with the
Perpendicular to the reflecting or refracting Surface at the Point of
Incidence._

DEFIN. VI.

_The Sines of Incidence, Reflexion, and Refraction, are the Sines of the
Angles of Incidence, Reflexion, and Refraction._

DEFIN. VII

_The Light whose Rays are all alike Refrangible, I call Simple,
Homogeneal and Similar; and that whose Rays are some more Refrangible
than others, I call Compound, Heterogeneal and Dissimilar._ The former
Light I call Homogeneal, not because I would affirm it so in all
respects, but because the Rays which agree in Refrangibility, agree at
least in all those their other Properties which I consider in the
following Discourse.

DEFIN. VIII.

_The Colours of Homogeneal Lights, I call Primary, Homogeneal and
Simple; and those of Heterogeneal Lights, Heterogeneal and Compound._
For these are always compounded of the colours of Homogeneal Lights; as
will appear in the following Discourse.

_AXIOMS._

AX. I.

_The Angles of Reflexion and Refraction, lie in one and the same Plane
with the Angle of Incidence._

AX. II.

_The Angle of Reflexion is equal to the Angle of Incidence._

AX. III.

_If the refracted Ray be returned directly back to the Point of
Incidence, it shall be refracted into the Line before described by the
incident Ray._

AX. IV.

_Refraction out of the rarer Medium into the denser, is made towards the
Perpendicular; that is, so that the Angle of Refraction be less than the
Angle of Incidence._

AX. V.

_The Sine of Incidence is either accurately or very nearly in a given
Ratio to the Sine of Refraction._

Whence if that Proportion be known in any one Inclination of the
incident Ray, 'tis known in all the Inclinations, and thereby the
Refraction in all cases of Incidence on the same refracting Body may be
determined. Thus if the Refraction be made out of Air into Water, the
Sine of Incidence of the red Light is to the Sine of its Refraction as 4
to 3. If out of Air into Glass, the Sines ar