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 could 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
53