the order of cones is
reversed, as the above figure shows. Only the two marginal rays of the
beam are depicted.

If a screen be held anywhere nearer the lens than the place marked 1
there will be a whitish centre to the patch of light and a red and
orange fringe or border. Held anywhere beyond the region 2, the border
of the patch will be blue and violet. Held about 3 the colour will be
less marked than elsewhere, but nowhere can it be got rid of. Each point
of an object will be represented in the image not by a point but by a
coloured patch: a fact which amply explains the observed blurring and
indistinctness.

Newton measured and calculated the distance between the violet and red
foci--VR in the diagram--and showed that it was 1/50th the diameter of
the lens. To overcome this difficulty (called chromatic aberration)
telescope glasses were made small and of very long focus: some of them
so long that they had no tube, all of them egregiously cumbrous. Yet it
was with such instruments that all the early discoveries were made. With
such an instrument, for instance, Huyghens discovered the real shape of
Saturn's ring.

The defects of refractors seemed irremediable, being founded in the
nature of light itself. So he gave up his "glass works"; and proceeded
to think of reflexion from metal specula. A concave mirror forms an
image just as a lens does, but since it does so without refraction or
transmission through any substance, there is no accompanying dispersion
or chromatic aberration.

The first reflecting telescope he made was 1 in. diameter and 6 in.
long, and magnified forty times. It acted as well as a three or four
feet refractor of that day, and showed Jupiter's moons. So he made a
larger one, now in the library of the Royal Society, London, with an
inscription:

"The first reflecting telescope, invented by Sir Isaac Newton, and made
with his own hands."

This has been the parent of most of the gigantic telescopes of the
present day. Fifty years elapsed before it was much improved on, and
then, first by Hadley and afterwards by Herschel and others, large and
good reflectors were constructed.

The largest telescope ever made, that of Lord Rosse, is a Newtonian
reflector, fifty feet long, six feet diameter, with a mirror weighing
four tons. The sextant, as used by navigators, was also invented by
Newton.

The year after the plague, in 1667, Newton returned to Trinity College,
and there continued his exper