So far as we now know, 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.
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