actors. The "secondary spectrum," as it is
called, also interposes a barrier troublesome to surmount. True
achromatism cannot be obtained with ordinary flint and crown-glass; and
although in lenses of "Jena glass," outstanding colour is reduced to
about one-sixth its usual amount, their term of service is fatally
abridged by rapid deterioration. Nevertheless, a 13-inch objective of
the new variety was mounted at Koenigsberg in 1898; and discs of Jena
crown and flint, 23 inches across, were purchased by Brashear at the
Chicago Exhibition of 1893. An achromatic combination of three kinds of
glass, devised by Mr. A. Taylor[1633] for Messrs. Cooke of York, has
less serious drawbacks, but has not yet come into extensive use.
Meanwhile, in giant telescopes affected to the full extent by chromatic
aberration, such as the Lick and Yerkes refractors, the differences of
focal length for the various colours are counted by inches,[1634] and
this not through any lack of skill in the makers, but by the necessity
of the case. Embarrassing consequences follow. Only a small part of the
spectrum of a heavenly body, for instance, can be distinctly seen at one
time; and a focal adjustment of half an inch is required in passing from
the observation of a planetary nebula to that of its stellar nucleus. A
refracting telescope loses, besides, one of its chief advantages over a
reflector when its size is increased beyond a certain limit. That
advantage is the greater luminosity of the images given by it.
Considerably more light is transmitted through a glass lens than is
reflected from an equal metallic surface. But only so long as both are
of moderate dimensions. For the glass necessarily grows in thickness as
its area augments, and consequently stops a larger percentage of the
rays it refracts. So that a point at length arrives--fixed by the late
Dr. Robinson at a diameter a little short of 3 feet[1635]--where the
glass and the metal are, in this respect, on an equality; while above it
the metal has the advantage. And since silvered glass gives back
considerably more light than speculum metal, the stage of equalisation
with lenses is reached proportionately sooner where this material is
employed.[1636]

The most distinctive faculty of reflectors, however, is that of bringing
rays of all refrangibilities to a focus together. They are naturally
achromatic. None of the beams they collect are thrown away in
colour-fringes, obnoxious both in thems