Whether the telescope be of the first or last order of excellence, its light-grasping powers will be practically the same; there is therefore a great distinction to be drawn between the illuminating and defining power. The former, as we have seen, depends upon size (and subsidiarily upon polish), the latter depends upon the accuracy of the curvature of the surface.
FIG. 83.—Saturn and his moons (general view with a 3¾-inch object-glass.) ]
If the defining power be not good, even if the air be perfect, each increase of the magnifying power so brings out the defects of the image, that at last no details at all are visible, all outlines are blurred, or stellar character is lost.
The testing of a glass therefore refers to two different qualities which it should possess. Its quality as to material and the fineness of its polish should be such that the maximum of light shall be transmitted. Its quality, as to the curves, should be such that the rays passing through every part of its area shall converge absolutely to the same point, with a chromatic aberration not absolutely _nil_, but sufficient to surround objects with a faint violet light.
FIG. 84.—Details of the ring of Saturn observed by Trouvelot with the 26-inch Washington Refractor. ]
In close double stars therefore, or in the more minute markings of the sun, moon, or planets, we have tests of its defining power; and if this is equally good in the instruments examined, the revelations of telescopes as they increase in power are of the most amazing kind.
A 3¾-inch suffices to show Saturn with all the detail shown in Fig. 83, while Fig. 84 shows us the further minute structure of the rings which comes out when the planet is observed with an aperture of 26 inches.
In the matter of double stars, a telescope of 2 inches aperture, with powers varying from 60 to 100, should show the following stars double:—
Polaris. α Piscium. μ Draconis. γ Arietis. ρ Herculis. ζ Ursæ Majoris. α Geminorum. γ Leonis. ξ Cassiopeæ.
A 4-inch aperture, powers 80-120, reveals the duplicity of—
β Orionis. ε Hydræ. ε Boötis. ι Leonis. α Lyræ. ξ Ursæ Majoris. γ Ceti. δ Geminorum. σ Cassiopeæ. ε Draconis.
ε Arietis. 32 Orionis. λ Ophiuchi. 20 Draconis. κ Geminorum. ι Equulei. ξ Herculis. ξ Boötis.