ide attention, Huygens was completely victorious;
and Hooke, being unable to refute Huygens's arguments, exhibited such
irritability that he increased his already general unpopularity. All of
the arguments for and against the telescope sight are too numerous to
be given here. In contending in its favor Huygens pointed out that the
unaided eye is unable to appreciate an angular space in the sky less
than about thirty seconds. Even in the best quadrant with a plain sight,
therefore, the altitude must be uncertain by that quantity. If in place
of the plain sight a telescope is substituted, even if it magnify only
thirty times, it will enable the observer to fix the position to one
second, with progressively increased accuracy as the magnifying power
of the telescope is increased. This was only one of the many telling
arguments advanced by Huygens.

In the field of optics, also, Huygens has added considerably to science,
and his work, Dioptrics, is said to have been a favorite book with
Newton. During the later part of his life, however, Huygens again
devoted himself to inventing and constructing telescopes, grinding the
lenses, and devising, if not actually making, the frame for holding
them. These telescopes were of enormous lengths, three of his
object-glasses, now in possession of the Royal Society, being of 123,
180, and 210 feet focal length respectively. Such instruments,
if constructed in the ordinary form of the long tube, were very
unmanageable, and to obviate this Huygens adopted the plan of dispensing
with the tube altogether, mounting his lenses on long poles manipulated
by machinery. Even these were unwieldy enough, but the difficulties of
manipulation were fully compensated by the results obtained.

It had been discovered, among other things, that in oblique refraction
light is separated into colors. Therefore, any small portion of the
convex lens of the telescope, being a prism, the rays proceed to the
focus, separated into prismatic colors, which make the image thus formed
edged with a fringe of color and indistinct. But, fortunately for the
early telescope makers, the degree of this aberration is independent of
the focal length of the lens; so that, by increasing this focal length
and using the appropriate eye-piece, the image can be greatly magnified,
while the fringe of colors remains about the same as when a less
powerful lens is used. Hence the advantage of Huygens's long telescope.
He did not confine