of the Spectrum, or
Confine of green and blue, but with that which verged a little more to
the blue than to the green: And as I reckoned the whole Length of the
Colours not to be the whole Length of the Spectrum, but the Length of
its Rectilinear Sides, so compleating the semicircular Ends into
Circles, when either of the observed Colours fell within those Circles,
I measured the distance of that Colour from the semicircular End of the
Spectrum, and subducting half this distance from the measured distance
of the two Colours, I took the Remainder for their corrected distance;
and in these Observations set down this corrected distance for the
difference of the distances of their Foci from the Lens. For, as the
Length of the Rectilinear Sides of the Spectrum would be the whole
Length of all the Colours, were the Circles of which (as we shewed) that
Spectrum consists contracted and reduced to Physical Points, so in that
Case this corrected distance would be the real distance of the two
observed Colours.

When therefore I farther observed the deepest sensible red, and that
blue whose corrected distance from it was 7/12 Parts of the Length of
the Rectilinear Sides of the Spectrum, the difference of the distances
of their Foci from the Lens was about 3-1/4 Inches, and as 7 to 12, so
is 3-1/4 to 5-4/7.

When I observed the deepest sensible red, and that indigo whose
corrected distance was 8/12 or 2/3 of the Length of the Rectilinear
Sides of the Spectrum, the difference of the distances of their Foci
from the Lens, was about 3-2/3 Inches, and as 2 to 3, so is 3-2/3 to
5-1/2.

When I observed the deepest sensible red, and that deep indigo whose
corrected distance from one another was 9/12 or 3/4 of the Length of the
Rectilinear Sides of the Spectrum, the difference of the distances of
their Foci from the Lens was about 4 Inches; and as 3 to 4, so is 4 to
5-1/3.

When I observed the deepest sensible red, and that Part of the violet
next the indigo, whose corrected distance from the red was 10/12 or 5/6
of the Length of the Rectilinear Sides of the Spectrum, the difference
of the distances of their Foci from the Lens was about 4-1/2 Inches, and
as 5 to 6, so is 4-1/2 to 5-2/5. For sometimes, when the Lens was
advantageously placed, so that its Axis respected the blue, and all
Things else were well ordered, and the Sun shone clear, and I held my
Eye very near to the Paper on which the Lens cast the Species of the
Lines, I cou