On the Size of Telescopic Star -Disks. 277 



also that I had come to the same conclusion with reference to 

 the ratio of the focal length to the aperture. Except the 

 difference of the curves, and of the ratio of the focal length 

 to the aperture, all other things were supposed to be alike. 

 The magnifying power, for instance, being as nearly as possible 

 the same. The outstanding chromatic dispersion produces no 

 effect on the size of the disk, but only on its colour, which is 

 of course very perceptible under high magnifiers: 



" The application of a circular patch to the centre of an 

 object-glass has been frequently employed in my double-star 

 observations since the year 1831 ; having been suggested to 

 me by Sir John Herschel, in a letter dated in that year. It is 

 also mentioned in the introduction to his Measurements of 

 364 Double Stars, published in 1832, where he says, ' The 

 action of a telescope is often surprisingly improved by stopping 

 out the central rays by a round disk from a fifth to a sixth of 

 the diameter of the object-glass, which should be well 

 centred.'' See Memoirs of Royal Astronomical Society, Vol. v., 

 Part i., p. 48. And in the introduction to my first series of 

 double star measures, I have recorded my use of it, and have 

 remarked, c The effect is decidedly good on the stars them- 

 selves, if not too faint to bear the loss of light. The separat- 

 ing power of the telescope is increased, but the concentric 

 rings accompanying bright stars are multiplied, and rendered 

 more luminous, and are also thrown further from the disk. 

 Hence small stars may often be obscured or distorted by the 

 ring passing through them/ [Memoirs of Royal Astronomical 

 Society, Yol. viii. p. 63.) 



"The effect of the different degrees of transparency in 

 object-glasses, arising from the different colour of the glass, is 

 very small comparatively, as may easily be proved by compar- 

 ing together the separating power of a refractor with that of 

 a good Newtonian (metal) reflector of the same aperture ; 

 the refractor having a central disk on the object-glass of the 

 same size as the plane mirror of the Newtonian. The reflector 

 will not have more than half the illuminating power of the re- 

 fractor, yet the difference in its separating power, arising from 

 the smaller size of the disks, will be but trifling, much the 

 same in fact as if the close stars of £ Gancri were both of the 

 size of the smaller of the two. 



" To assert, as Dr. Steinheil supposed I did, that with a 

 given aperture, the disks of all stars, whether of the 1st or 

 the 9th magnitude, would appear of the same size, would be 

 simply absurd. It appears that he overlooked the limitation 

 distinctly expressed by the President in the words, ' With 

 regard to the size of the spurious disk of the same star/ etc., 

 and it was to this that my reply referred. — I am, sir, your 

 obedient servant, "W.R.Dawes." 



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