t PLANET JUNO. * 293 



tnat an increafe of light might now be of great ufe, and Obfervations on 

 viewed the afteroid with a fine 10-feet mirror of 24. inches *h c P lanftt J 1 " 1 '* 

 diameter, but found that nothing was gained by the change. 

 The temperature indeed of thefe large mirrors is very feldom 

 the fame as that of the air in which they are to act, and till a 

 perfect nniformity takes place no high powers can be u fed. 



The afteroid in the meridian, and the night beautiful. After 

 many repeated comparifons of equal liars with the afteroid, I 

 think it (hows more of a difk than they do, but it is fo fmall 

 that it cannot amount to fo much as 3-tenths of a fecond, or 

 at leaft to no more. r - ' 



It is accompanied with rather more nebulofity than ftars of 

 the fame fize. 



The night is fo clear, that I cannot fuppofe vifion at this 

 altitude to be lefs perfect on the ftars, than it is on day ob- 

 jects at the diftance of 800 feet in a direction almoft hori- 

 zontal. x 



Oct. U. By comparing the afteroid alternately and often, 

 with equal ftars, its di/k, if it be a real one, cannot exceed 2, 

 or at moft 3-tenths of a fecond. This eftimation is founded 

 on the comparative readinefs with which every fine day I have 

 feen globules fubtending fuch angles in the fame telefeope, 

 and with the fame magnifying power. 



*' The afteroid is in the meridian, and in high perfection. 

 I perceive a well defined difk that may amount to 2 or 3-tenths ,_, H 



of a fecond ; but an equal ftar fhows exactly the fame ap- 

 pearance, and has a difk as well defined and as large as that 

 of the afteroid." 



Refuk and Application of the Experiments and Obfervations. 



We may now proceed to draw a few very ufeful concluflons 

 from the experiments that have been given, and apply them 

 to the obfervations of the ftar difcovered by Mr. Harding ; 

 and alfo to the fimilar ftars of Mr. Piazzi and Dr. Olbers. 

 Thefe kind of corollaries may be exprefled as follows. 



(1.) A 10-feet reflector will fhew the fpurious or real 

 difks of celeftial and terreftrial objects, when their diameter 

 is | of a fecond of a degree ; and when every circumftance 

 js favourable, fuch a diameter may be perceived fo diftinclly, 

 that it can be divided bv eftimation into two or three parts. 



(2.) A 



