ON TWO CELESTIAL BODIES. J21 



angles to the telefcope, far enough to make it appear no larger 

 than the ftar ; and, not having diiks of a lefs diameter prepared, 

 I placed the imalleft I had as far from me as the fituation of 

 the fiar would allow. Then, bringing its image again by the 

 fide of the difk, and viewing, at the fame time, with one eve 

 the magnified liar, while the other eye faw the lucid dilk, I 

 perceived that Ceres, which is the name the difcoverer has 

 given to the ftar, was hardly more than one third of the diame- 

 ter of the dilk, and certainly lefs than one half of it. 



This being repeated, and always appearing the fame, we 

 (hall not under-rate the (ize of the flar, by admitting its dia- ' 

 meter to have been 45 hundredths of the lucid difk. 



The power of the telefcope, very precifely afcertained by Power 370,4*. 

 terreftrial geometrical meafures properly reduced to the focus 

 of the mirror on the ftars, was 370,42. The diftance of the 

 lucid difk from the eye, was 2131 inches; and its diameter 3,4 

 inches. Hence we compute, that the difk was feen under an 

 angle of 5' 29 // ,09 ; and Ceres, when magnified 370 times, 

 appearing, as we have fhewn, 45 hundredths of that magni- Diameter of Ce* 

 tude, its real diameter could not exceed 0' ; ,40. Had this dia- res ° '* * 

 meter amounted to as much as was formerly eftimated, the 

 power of 370 would have made it appear of 6 / 10", which is 

 more than the whole lucid difk. 



This extraordinary refult raifed in me a fufpicion, that the 

 power 370 of a 7-feet telefcope, and its aperture of 6,3 inches, 

 might not be fufficient to mew the planet's feeble light pro- 

 perly. I therefore adapted my 10-feet inftrument toobferva- 

 tions with lucid difks ; which require a different arrangement 

 of the head of the telefcope and finder : I alfo made fome 

 fmall tranfparencies, to reprefent the object I intended to 

 meafure. 



-April 21. The night being pretty clear, though perhaps a. Obfervatlon 

 not quite fo proper for delicate vifion as I could have wifhed, ^"^.fee^Te! 

 I directed my 10-feet reflector, with a magnifying power of fle&or. Power 

 516,54, alfo afcertained by geometrical terreftriai meafures 5 16 > 54* 

 reduced to the focus of the inftrument on celeftial objects, to 

 Mr. Piazzas flar, and compared it with a lucid difk, placed at 

 1436 inches from the eye, and of 1,4 inch in diameter. I varied 

 the diftance of the lucid difk many tiroes ; and fixed at laft on 

 the above-mentioned one, as the befi I could find. There was, 



however t 



