h Part of the distance of the
Focus of the mean refrangible Rays from the Lens, as the distance
between that Focus and the lucid Point, from whence the Rays flow, is to
the distance between that lucid Point and the Lens very nearly.

Now to examine whether the Difference between the Refractions, which the
most refrangible and the least refrangible Rays flowing from the same
Point suffer in the Object-glasses of Telescopes and such-like Glasses,
be so great as is here described, I contrived the following Experiment.

_Exper._ 16. The Lens which I used in the second and eighth Experiments,
being placed six Feet and an Inch distant from any Object, collected the
Species of that Object by the mean refrangible Rays at the distance of
six Feet and an Inch from the Lens on the other side. And therefore by
the foregoing Rule, it ought to collect the Species of that Object by
the least refrangible Rays at the distance of six Feet and 3-2/3 Inches
from the Lens, and by the most refrangible ones at the distance of five
Feet and 10-1/3 Inches from it: So that between the two Places, where
these least and most refrangible Rays collect the Species, there may be
the distance of about 5-1/3 Inches. For by that Rule, as six Feet and an
Inch (the distance of the Lens from the lucid Object) is to twelve Feet
and two Inches (the distance of the lucid Object from the Focus of the
mean refrangible Rays) that is, as One is to Two; so is the 27-1/2th
Part of six Feet and an Inch (the distance between the Lens and the same
Focus) to the distance between the Focus of the most refrangible Rays
and the Focus of the least refrangible ones, which is therefore 5-17/55
Inches, that is very nearly 5-1/3 Inches. Now to know whether this
Measure was true, I repeated the second and eighth Experiment with
coloured Light, which was less compounded than that I there made use of:
For I now separated the heterogeneous Rays from one another by the
Method I described in the eleventh Experiment, so as to make a coloured
Spectrum about twelve or fifteen Times longer than broad. This Spectrum
I cast on a printed Book, and placing the above-mentioned Lens at the
distance of six Feet and an Inch from this Spectrum to collect the
Species of the illuminated Letters at the same distance on the other
side, I found that the Species of the Letters illuminated with blue were
nearer to the Lens than those illuminated with deep red by about three
Inches, or three and a quarte