he sun's discus, so as to
differ 31' in their obliquity of incidence, and found that the emergent
rays should have comprehended an angle of 31', as they did, before they
were incident.

"But because this computation was founded on the hypothesis of the
proportionality of the sines of incidence and refraction, which though
by my own experience I could not imagine to be so erroneous as to make
that angle but 31', which in reality was 2 degrees 49', yet my curiosity
caused me again to make my prism. And having placed it at my window,
as before, I observed that by turning it a little about its axis to and
fro, so as to vary its obliquity to the light more than an angle of 4
degrees or 5 degrees, the colors were not thereby sensibly translated
from their place on the wall, and consequently by that variation of
incidence the quantity of refraction was not sensibly varied. By this
experiment, therefore, as well as by the former computation, it was
evident that the difference of the incidence of rays flowing from divers
parts of the sun could not make them after decussation diverge at a
sensibly greater angle than that at which they before converged; which
being, at most, but about 31' or 32', there still remained some other
cause to be found out, from whence it could be 2 degrees 49'."

All this caused Newton to suspect that the rays, after their trajection
through the prism, moved in curved rather than in straight lines, thus
tending to be cast upon the wall at different places according to the
amount of this curve. His suspicions were increased, also, by happening
to recall that a tennis-ball sometimes describes such a curve when "cut"
by a tennis-racket striking the ball obliquely.

"For a circular as well as a progressive motion being communicated to
it by the stroke," he says, "its parts on that side where the motions
conspire must press and beat the contiguous air more violently than
on the other, and there excite a reluctancy and reaction of the air
proportionately greater. And for the same reason, if the rays of light
should possibly be globular bodies, and by their oblique passage out of
one medium into another acquire a circulating motion, they ought to feel
the greater resistance from the ambient ether on that side where the
motions conspire, and thence be continually bowed to the other. But
notwithstanding this plausible ground of suspicion, when I came to
examine it I could observe no such curvity in them. And,