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REPORTS FROM THE SECTIONS. 241 
On Star-discs and the separating power of Telescopes. 
By W. J. MacDonnett, F.R.A.S. 
[Read before the Astronomical Section, 4 October, 1878.] 
appear as mere points of light, no telescope, however large, no 
magnifying power however great, has ever raised a real disc like 
that of the planets. It is, nevertheless, a well known conse- 
quence of optical laws, explainable by the undulatory theory of 
light, that the telescope shows the stars with a sensible disc of a 
varying size according to the nature, quality, and power of the 
or focus or the relation of aperture to focus; and with the view 
lackness, surrounded by a series of alternate bright and dark 
rings, the intensities of the bright rings being respectively about 
v1) zho, why of that of the central disc. Two well known 
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“Separate stars of the sixth magnitude 4”-6 apart, and hence the 
Separating power of: any given aperture “a” will be expressed by 
the fraction *=° (Monthly Notices, Vol. 27, pp. 217-238. See 
also “ Astronomical Register,” vol. 3, p. 153, and vol. 5, p. 100 ; 
itmiery Observer,” vol. 8, pp. 240 and 276 ; Smyth’s “Cycle, 
vol I, p. 417), : 
Mr. G. Knott, who is an experienced observer, and is the- 
Possessor of a fine 7}-inch Alvan Clark refractor, instituted a. 
Series of experiments for the direct measurement of the telescopic: 
discs of stars, employing various apertures of his telescope for 
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