276 On the Size of Telescopic Star-Disks. 



tion of the ink, which is pretty considerable, also exactly similar. 

 On first turning up the sketch, I had some doubt on this point, my 

 habit of dating during the latter part of my residence at the Cape 

 being " Feldhausen," the name of my residence, an insulated house 

 near Wynberg ; but from the subjoined extract of a letter from 

 Captain Horsburgh,* I find that I was occasionally also in the habit 

 of dating from Wynberg. 



I have the honour, etc., etc., sir, your obedient servant, 



J. F. W. Herschbl. 



P.S. — The roof proposed being light, no great strength of cable 

 would be necessary. A series of wire ropes, like those which sup- 

 ported the old suspension bridge over the Rhone at Valence, would 

 answer every purpose, and be very cheap. 



ON THE SIZE OP TELESCOPIC STAR-DISKS. 



BY THE REV. W. E. DAWES. 



We have much pleasure in laying before our readers the follow- 

 ing important letter from Mr. Dawes, on a subject now exciting 

 very general attention : — 



" The remarks in the Intellectual Observer of July and 

 October, on the little discussion which has arisen respecting 

 the size of telescopic star-disks, seemed to show that the 

 object of my statements on the subject was in some respects 

 misapprehended. 



" My remarks at the June meeting of the Royal Astrono- 

 mical Society had specific reference to two points on which it 

 was desired to ascertain the results of careful experiments — 

 namely : — 



" ' 1. Is the size of telescopic disks influenced by the 

 different curves used by opticians for the correction of the 

 spherical aberration of an object-glass ? 



" ' 2. Is it affected by the ratio of the focal length to the 

 aperture V 



" It was distinctly understood that the spherical aberration 

 was well corrected, though by different curves, in all the object- 

 glasses compared ; and that the observations were made on 

 the same star. My reply to the questions expressed simply, 

 that the results of numerous comparisons which I had made 

 with telescopes by the eight different opticians named, had 

 proved that the variety of the curves employed by them pro- 

 duced no perceptible difference in the size of the disk, and 



* (EXTRACT.) 



" My dear Sir,— Your valuable and esteemed letter, dated Wynberg, July 

 7, 1834, etc., etc. 



(Signed) " Jas. Uorsburgii. 



"Sir J. F. W. Herschel, Wynberg, 



" Cape Town, Capo of Good Hope." 



