a good three-inch telescope,
nay, a first-rate two inch one, will show far more than our
great-grandfathers ever saw, or dreamed of seeing, with their
refractors.

Toward the middle of the seventeenth century the reflecting telescope
had been so much improved as nearly to crowd out its refracting rival,
but, just as its success seemed to be assured, Dollond, working along
lines partially followed up by Hall, found a combination of lenses by
which the chromatic aberration of the refractor could be very
perfectly corrected. While Dollond's invention was of immense value,
it remained that flint object glasses larger than two and one-half
inches in diameter could not, for some years, be manufactured, but
about the opening of the nineteenth century, Guinand, a Swiss,
discovered a process of making masses of optical flint glass
sufficiently large as to admit of the construction from them of
excellent lenses of sizes gradually increasing as time and
experimenting went on. The making of three-inch objectives, achromatic
and of short focus, wrought a revolution in telescopes and renewed the
demand for refractors, though prices, as compared with those of the
present day, were very great. But improvement was succeeded by
improvement. Larger and still larger objectives were made, yet
progress was not so rapid as not to justify Grant, in 1852, in
declaring to be a "munificent gift" the presentation, about 1838, to
Greenwhich Observatory, of a six and seven-tenths object glass alone,
and so it was esteemed by Mr. Airy, the astronomer royal. Improvement
is still the order of the day, and, as a result of keen competition,
very excellent telescopes of small aperture can be purchased at
reasonable prices. Great telescopes are enormously expensive, and will
probably be so until they are superseded by some simple invention
which shall be as superior to them as they are to the "mighty"
instruments which, from time to time, caused such sensations in the
days of Galilei, Cassini, Huyghens, Bradley, Dollond, and those who
came after them.

But, notable as are the services rendered to science by giant
telescopes, it remains that by far the greater bulk of useful work has
been done by apertures of less than twelve inches in diameter. Indeed,
it may be asserted that most of such work has been done by instruments
of six inches or less in size. After referring with some detail to
this, Denning tells us that "nearly all the comets, planetoids, d