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
several sorts of Rays, hold the same Proportion to one another in the
smallest Refractions which they do in the greatest.
My Progress in making this nice and troublesome Experiment I have set
down more at large, that they that shall try it after me may be aware of
the Circumspection requisite to make it succeed well. And if they cannot
make it succeed so well as I did, they may notwithstanding collect by
the Proportion of the distance of the Colours of the Spectrum, to the
Difference of the distances of their Foci from the Lens, what would be
the Success in the more distant Colours by a better trial. And yet, if
they use a broader Lens than I did, and fix it to a long strait Staff,
by means of which it may be readily and truly directed to the Colour
whose Focus is desired, I question not but the Experiment will succeed
better with them than it did with me. For I directed the Axis as nearly
as I could to the middle of the Colours, and then the faint Ends of the
Spectrum being remote from the Axis, cast their Species less distinctly
on the Paper than they would have done, had the Axis been successively
directed to them.
Now by what has been said, it's certain that the Rays which differ in
54