Dec. 13 1877] 



NA TURE 



125 



THE MODERN TELESCOPE^ 



II. 



'HETHER 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. 



W 



Fig. 5.— Saturn and his moons (general view with ajfinch object-glass.) 



The former as we have seen depends upon size (and sub- 

 sidiarily upon polish), the latter depends upon the accuracy 

 of the curvature of the surface. 



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. Even with the best telescopes the power should not 

 be strained. 



The testing of a glass therefore refers to two different 

 qualities which it should possess. Its quality as to ma- 

 terial 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 wiih a faint violet 

 light. With the reflector we have to consider the brilliancy 

 of the surface and the perfection of curvature. 



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 3|-inch suffices to show Saturn with all the detail 

 shown in Fig. 5, while Fig. 6 shows us the further 

 minute structure of the rings wh'ch coties out when the 

 planet is observed with an object-glass with an aperture of 

 26 inches. 



In the matter 01 double stars, a telescope of 2 inches 



Fig. 6.— Details of the ring of Saturn observed by Trouvelot wit'i ihe 26-inch Washington Refractor. 



aperture, with powers varying from 60 to 100, should 



show the following stars double : — 



Polaiis. 7 Arietis. a Geminorum. 



o Piscium. p Herculis. 7 Leonis, 



)u Draconis. f Ursae Majoris. \ Cassiopeop. 



A 4-inch aperture, powers 80-120, reveals the duplicity 



of — 



/3 Orionis. a Lyrse. 8 Geminorum. 



e Hydrre. f Urise Majoris. a Cassiopeae. 



6 Booti<. 7 Ceti. « Draconis. 



I Leon's. 



' Continued from p. 68. 



A 6- inch, powers 240-300 — 



6 Ar etis. 20 Draconis. 



32 Orionis. « GemiiiOrum. 



A Ophiuchi. * EquuleL 



An 8- inch — 



8 Cygni Siriu«. 



7'^ Andromec'se. 19 Draconis. 



The " spurious disk," which a fixed star presents, as 

 seen in the telescope, is an effect which results from the 

 passage of the light through the circular object-glass, 

 or its reflection Irom a circular mirror ; and it is this 



U 2 



^ Herculis. 

 \ Bootis. 



y? Herculis. 

 ti? Bootis. 



