and be
refracted before it was transmitted through the hole; but I found none
of those circumstances material. The fashion of the colors was in all
these cases the same.

"Then I suspected whether by any unevenness of the glass or other
contingent irregularity these colors might be thus dilated. And to try
this I took another prism like the former, and so placed it that the
light, passing through them both, might be refracted contrary ways,
and so by the latter returned into that course from which the former
diverted it. For, by this means, I thought, the regular effects of the
first prism would be destroyed by the second prism, but the irregular
ones more augmented by the multiplicity of refractions. The event was
that the light, which by the first prism was diffused into an oblong
form, was by the second reduced into an orbicular one with as much
regularity as when it did not all pass through them. So that, whatever
was the cause of that length, 'twas not any contingent irregularity.

"I then proceeded to examine more critically what might be effected by
the difference of the incidence of rays coming from divers parts of the
sun; and to that end measured the several lines and angles belonging to
the image. Its distance from the hole or prism was 22 feet; its utmost
length 13 1/4 inches; its breadth 2 5/8; the diameter of the hole 1/4
of an inch; the angle which the rays, tending towards the middle of the
image, made with those lines, in which they would have proceeded without
refraction, was 44 degrees 56'; and the vertical angle of the prism, 63
degrees 12'. Also the refractions on both sides of the prism--that is,
of the incident and emergent rays--were, as near as I could make
them, equal, and consequently about 54 degrees 4'; and the rays fell
perpendicularly upon the wall. Now, subducting the diameter of the hole
from the length and breadth of the image, there remains 13 inches
the length, and 2 3/8 the breadth, comprehended by those rays, which,
passing through the centre of the said hole, which that breadth
subtended, was about 31', answerable to the sun's diameter; but the
angle which its length subtended was more than five such diameters,
namely 2 degrees 49'.

"Having made these observations, I first computed from them the
refractive power of the glass, and found it measured by the ratio of the
sines 20 to 31. And then, by that ratio, I computed the refractions
of two rays flowing from opposite parts of t