[27] Such apparatus is essentially appurtenant to large instruments only, say of not less than 12″ aperture and preferably much more. The eye is enormously more sensitive as a detector of radiant energy than any device of human contrivance, and thus small telescopes can be well used for visual photometry, the bigger instruments having then merely the advantage of reaching fainter stars.
[28] E. g., the beautiful astrographic and other objectives turned out by the brothers Henry.
[29] This and several of the subsequent figures are taken from quite the best account of testing objectives: “On the Adjustment and Testing of Telescope Objectives.” T. Cooke & Sons, York, 1891, a little brochure unhappily long since out of print. A new edition is just now, 1922, announced.
[30] Sometimes with ever so careful centering the ring system in the middle of the field is still eccentric with respect to the small mirror, showing that the axis of the parabola is not perpendicular to the general face of the mirror. This can usually be remedied by the adjusting screws of the main mirror as described, but now and then it is necessary actually to move over the small mirror into the real optical axis. Draper (loc. cit.) gives some experiences of this sort.
[31] See also two valuable papers by Sir Howard Grubb, The Observatory, Vol. VII, pp. 9, 43. Also in Jour. Roy. Ast. Soc. Canada, Dec., 1921, Jan. 1922.
Transcriber's Notes
Obvious typographical errors have been silently corrected. Variations in hyphenation and accents have been standardised but all other spelling and punctuation remains unchanged.
In caption of [Fig. 49].—Spherical Aberration of Concave Lens. Concave has been changed to Convex
In “[An objective of 4.56′ inches] aperture has a resolving constant of 1″ and to develop this should take a magnification of say 300,” 1″ has been hand altered in the original and may be 1′.
The table “[Characteristics of Optical Glasses]″ has been divided to fit within the width restriction.