These are the Laws of the Refractions made out of Glass into Air, and
thence by the third Axiom of the first Part of this Book, the Laws of
the Refractions made out of Air into Glass are easily derived.
_Exper._ 8. I found moreover, that when Light goes out of Air through
several contiguous refracting Mediums as through Water and Glass, and
thence goes out again into Air, whether the refracting Superficies be
parallel or inclin'd to one another, that Light as often as by contrary
Refractions 'tis so corrected, that it emergeth in Lines parallel to
those in which it was incident, continues ever after to be white. But if
the emergent Rays be inclined to the incident, the Whiteness of the
emerging Light will by degrees in passing on from the Place of
Emergence, become tinged in its Edges with Colours. This I try'd by
refracting Light with Prisms of Glass placed within a Prismatick Vessel
of Water. Now those Colours argue a diverging and separation of the
heterogeneous Rays from one another by means of their unequal
Refractions, as in what follows will more fully appear. And, on the
contrary, the permanent whiteness argues, that in like Incidences of the
Rays there is no such separation of the emerging Rays, and by
consequence no inequality of their whole Refractions. Whence I seem to
gather the two following Theorems.
1. The Excesses of the Sines of Refraction of several sorts of Rays
above their common Sine of Incidence when the Refractions are made out
of divers denser Mediums immediately into one and the same rarer Medium,
suppose of Air, are to one another in a given Proportion.
2. The Proportion of the Sine of Incidence to the Sine of Refraction of
one and the same sort of Rays out of one Medium into another, is
composed of the Proportion of the Sine of Incidence to the Sine of
Refraction out of the first Medium into any third Medium, and of the
Proportion of the Sine of Incidence to the Sine of Refraction out of
that third Medium into the second Medium.
By the first Theorem the Refractions of the Rays of every sort made out
of any Medium into Air are known by having the Refraction of the Rays of
any one sort. As for instance, if the Refractions of the Rays of every
sort out of Rain-water into Air be desired, let the common Sine of
Incidence out of Glass into Air be subducted from the Sines of
Refraction, and the Excesses will be 27, 27-1/8, 27-1/5, 27-1/3, 27-1/2,
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