nts, I
measured from the faintest and outmost red at one end, to the faintest
and outmost blue at the other end, excepting only a little Penumbra,
whose breadth scarce exceeded a quarter of an Inch, as was said above.

_Exper._ 4. In the Sun's Beam which was propagated into the Room through
the hole in the Window-shut, at the distance of some Feet from the hole,
I held the Prism in such a Posture, that its Axis might be perpendicular
to that Beam. Then I looked through the Prism upon the hole, and turning
the Prism to and fro about its Axis, to make the Image of the Hole
ascend and descend, when between its two contrary Motions it seemed
Stationary, I stopp'd the Prism, that the Refractions of both sides of
the refracting Angle might be equal to each other, as in the former
Experiment. In this situation of the Prism viewing through it the said
Hole, I observed the length of its refracted Image to be many times
greater than its breadth, and that the most refracted part thereof
appeared violet, the least refracted red, the middle parts blue, green
and yellow in order. The same thing happen'd when I removed the Prism
out of the Sun's Light, and looked through it upon the hole shining by
the Light of the Clouds beyond it. And yet if the Refraction were done
regularly according to one certain Proportion of the Sines of Incidence
and Refraction as is vulgarly supposed, the refracted Image ought to
have appeared round.

So then, by these two Experiments it appears, that in Equal Incidences
there is a considerable inequality of Refractions. But whence this
inequality arises, whether it be that some of the incident Rays are
refracted more, and others less, constantly, or by chance, or that one
and the same Ray is by Refraction disturbed, shatter'd, dilated, and as
it were split and spread into many diverging Rays, as _Grimaldo_
supposes, does not yet appear by these Experiments, but will appear by
those that follow.

_Exper._ 5. Considering therefore, that if in the third Experiment the
Image of the Sun should be drawn out into an oblong Form, either by a
Dilatation of every Ray, or by any other casual inequality of the
Refractions, the same oblong Image would by a second Refraction made
sideways be drawn out as much in breadth by the like Dilatation of the
Rays, or other casual inequality of the Refractions sideways, I tried
what would be the Effects of such a second Refraction. For this end I
ordered all things as in the th