be readily found by the Method in the following Proposition.

_PROP._ VII. THEOR. VI.

_The Perfection of Telescopes is impeded by the different Refrangibility
of the Rays of Light._

The Imperfection of Telescopes is vulgarly attributed to the spherical
Figures of the Glasses, and therefore Mathematicians have propounded to
figure them by the conical Sections. To shew that they are mistaken, I
have inserted this Proposition; the truth of which will appear by the
measure of the Refractions of the several sorts of Rays; and these
measures I thus determine.

In the third Experiment of this first Part, where the refracting Angle
of the Prism was 62-1/2 Degrees, the half of that Angle 31 deg. 15 min.
is the Angle of Incidence of the Rays at their going out of the Glass
into the Air[F]; and the Sine of this Angle is 5188, the Radius being
10000. When the Axis of this Prism was parallel to the Horizon, and the
Refraction of the Rays at their Incidence on this Prism equal to that at
their Emergence out of it, I observed with a Quadrant the Angle which
the mean refrangible Rays, (that is those which went to the middle of
the Sun's coloured Image) made with the Horizon, and by this Angle and
the Sun's altitude observed at the same time, I found the Angle which
the emergent Rays contained with the incident to be 44 deg. and 40 min.
and the half of this Angle added to the Angle of Incidence 31 deg. 15
min. makes the Angle of Refraction, which is therefore 53 deg. 35 min.
and its Sine 8047. These are the Sines of Incidence and Refraction of
the mean refrangible Rays, and their Proportion in round Numbers is 20
to 31. This Glass was of a Colour inclining to green. The last of the
Prisms mentioned in the third Experiment was of clear white Glass. Its
refracting Angle 63-1/2 Degrees. The Angle which the emergent Rays
contained, with the incident 45 deg. 50 min. The Sine of half the first
Angle 5262. The Sine of half the Sum of the Angles 8157. And their
Proportion in round Numbers 20 to 31, as before.

From the Length of the Image, which was about 9-3/4 or 10 Inches,
subduct its Breadth, which was 2-1/8 Inches, and the Remainder 7-3/4
Inches would be the Length of the Image were the Sun but a Point, and
therefore subtends the Angle which the most and least refrangible Rays,
when incident on the Prism in the same Lines, do contain with one
another after their Emergence. Whence this Angle is 2 deg. 0'. 7''. For
the distance betw