Magnitude of smal£ celestial bodies* 285 



xvur. 



Br.perbnents for afcertaining how far Ttkfcopes will enable ua 

 to determine very J mall Angles, and to difiinguijli the real from 

 thefpurious Diameters of celejiial and terrejirial Objecls : with 

 an Application of the llefult of thefe Experiments to a Series of 

 Obferzations on the Nature and Magnitude, of Mr .Harding's 

 lately dif covered Star. By William Herschel, L% L D. 

 F. R.S. Abridged from the Phil. Tranf 1805. 



JL HE difcovery of Mr. Harding having added a moving Enquiry as to 



celeaial body to the lift of thole that were known before, Dr. ^g,^^ 



Herfchel was ddirous of afcertainingits magnitude : and as which the eye 



in the observations .which it was neceflary to make he intended b > a telefco P e 



... can determine 



chiefly to ufe a ten -feet reflector, it appeared to be a defidera- the figure of an 



4um highly worthy of investigation to determine how fmall a ob i ec ^ 



diameter of an object might be feen by this inftrument. It is 



known that a very thin line may be perceived, and that objects 



may be feen when they fubtend a very fmall angle ; but the 



Cafe he wanted to determine related to a vifible dilk ; a round, 



well defined appearance, which might without hefitation be 



affirmed to be circular, if not fpherical. 



In April of the year 1774, the Doctor determined a iimilar The author's 



queflion relating to the natural eye : and found that a fquare unaffifte< ! e ye 

 u l j'/i- r cannot diftin- 



area could not be diltinguifhed from an equal circular one till guiih a finally 



the diameter of the latter came to fubtend an angle of 2' 17". ci / cle ,* aii ^ f 



He did not think it right to apply the fame conclufions to a equal Jqofce* * 



telefcopic view of an object, and therefore had recourfe to the 



a feries of experiments. 



The firft courfe of experiment, was made with the heads of Experiments 



pins deprived of their poiifh by tarnifhing them in the flame of ^\ ! h * h g qJ. 



a candle. The diameters of the heads were meafured by a jetts were pin's 



microfcopic projection, with a magnifying power of 80. Thefe heads * 



meafures were fo exact, than when repeated they feldom dif. 



fered more than a few ten thoufandths part of an inch from 



each other. The focal length of the mirror on Arcturus is 



119,64 inches, but on thefe objects 125,9, and the difiance 



was meafured with deal rods. 



And the refult of this experiment was that on object having 



a diameter ,0425 could be eafily feen in the author's ten-foot 



telefcope to be a round body, when the magnified angle under 



which 



