Note 54 — Chapter XVI · THE STARS.
Reading edition
Mr. George Knott, of Cuckfield, mentions that the radius of the first bright diffraction-ring of a stellar image, for a 7-1/3-inch aperture, is 1″·01, and for one of 2 inches 3″·70 (‘Observatory,’ vol. vi. p. 19; see also vol. i. pp. 107 and 145). Mr. Dawes is quoted as giving 1″·25 for a 7-inch, 1″·61 for a 5½-inch, and 3″·57 for a 2·4-inch. These figures exceed the theoretical values, if the latter are adopted from Sir G. B. Airy’s ‘Undulatory Theory of Optics,’ where, for mean rays, we have:—
Radius of object-glass in inches × radius of bright ring in seconds = 3·70.