ld see pretty distinctly the Species of those Lines by that
Part of the violet which was next the indigo; and sometimes I could see
them by above half the violet, For in making these Experiments I had
observed, that the Species of those Colours only appear distinct, which
were in or near the Axis of the Lens: So that if the blue or indigo were
in the Axis, I could see their Species distinctly; and then the red
appeared much less distinct than before. Wherefore I contrived to make
the Spectrum of Colours shorter than before, so that both its Ends might
be nearer to the Axis of the Lens. And now its Length was about 2-1/2
Inches, and Breadth about 1/5 or 1/6 of an Inch. Also instead of the
black Lines on which the Spectrum was cast, I made one black Line
broader than those, that I might see its Species more easily; and this
Line I divided by short cross Lines into equal Parts, for measuring the
distances of the observed Colours. And now I could sometimes see the
Species of this Line with its Divisions almost as far as the Center of
the semicircular violet End of the Spectrum, and made these farther
Observations.

When I observed the deepest sensible red, and that Part of the violet,
whose corrected distance from it was about 8/9 Parts of the Rectilinear
Sides of the Spectrum, the Difference of the distances of the Foci of
those Colours from the Lens, was one time 4-2/3, another time 4-3/4,
another time 4-7/8 Inches; and as 8 to 9, so are 4-2/3, 4-3/4, 4-7/8, to
5-1/4, 5-11/32, 5-31/64 respectively.

When I observed the deepest sensible red, and deepest sensible violet,
(the corrected distance of which Colours, when all Things were ordered
to the best Advantage, and the Sun shone very clear, was about 11/12 or
15/16 Parts of the Length of the Rectilinear Sides of the coloured
Spectrum) I found the Difference of the distances of their Foci from the
Lens sometimes 4-3/4 sometimes 5-1/4, and for the most part 5 Inches or
thereabouts; and as 11 to 12, or 15 to 16, so is five Inches to 5-2/2 or
5-1/3 Inches.

And by this Progression of Experiments I satisfied my self, that had the
Light at the very Ends of the Spectrum been strong enough to make the
Species of the black Lines appear plainly on the Paper, the Focus of the
deepest violet would have been found nearer to the Lens, than the Focus
of the deepest red, by about 5-1/3 Inches at least. And this is a
farther Evidence, that the Sines of Incidence and Refraction of the
s