NEW EDITION
OF
HINTS
ON
Silvered-Glass Reflecting
TELESCOPES
MANUFACTURED BY
MR. G. CALVER, F.R.A.S.
WITH DIRECTIONS FOR SILVERING, ADJUSTING, &c.
1877.
GEORGE CALVER,
HILL HOUSE, WIDFORD,
CHELMSFORD, ESSEX.
HINTS
ON
Silvered-Glass Reflecting Telescopes.
Of the various forms of Telescopes now in use, each has itsown peculiar advantages; but the Silvered-Glass Reflectoris undoubtedly gaining favour among our practicalobservers. A well-figured speculum, being perfectly freefrom chromatic aberration, gives, in a proper condition of theatmosphere, the finest possible definition of the Moon and planets,the markings and colours of these objects being excellently seen;while coloured stars, such as Albireo (β Cygni), or Almaach (γ Andromedæ),are exceedingly well shown, the beautiful contrast of thestars in the former being especially noticeable in a reflector. Theadvice of “F.R.A.S.” (in the “English Mechanic,” March 21st, 1873)as to the choice of a Telescope, may here appropriately be quoted.After expressing a preference for refractors when measuring closedouble-stars, he says, “But should the object of your correspondentbe merely to regard the wonders and beauties of the Heavens, ornotably, should he purpose to devote himself to the study of thephysical structure of the Moon and planets, then by all means let himobtain the largest reflector he can afford; its absolute achromatismtells most astonishingly on these last-named objects.” This is theopinion of one who has great practical knowledge of the differentforms of Telescopes.
If Achromatic Telescopes of large aperture could be made ascheaply as reflectors, and in as convenient a form, they woulddoubtless be preferred for general star-work, although the aberrations,especially that of colour, cannot be so perfectly corrected. Asilvered-glass reflector is, however, much cheaper than a refractor,and, when the aperture exceeds five or six inches, is much handier towork, and occupies less space, being only about half the length ofan achromatic of the same aperture. It is true that a reflector willgive less light than an achromatic of equal aperture—but this is, in[Pg 4]certain conditions of the atmosphere, a distinct advantage, the extraaperture to give the same light adding to the definition and penetratingpower. An example of this is seen in the beautiful definition givenby an unsilvered mirror on brilliant objects, as the Sun, Moon, andVenus. In large achromatics, the distressing excess of light has oftento be reduced by diminishing the aperture or using a higher powerthan is convenient; and in such cases a lower and more suitablepower can be employed with a reflector.
When the air is unsteady, the definition of Reflectors, owing totheir tubes being open, is more liable to fluctuate than that ofrefractors, although when a reflector does not give good