Planet juno. 289 



(9.) But that the change of the apertures a£ls more on the 

 fmall, and lefs on the large ones. 



(10.) The fpurious difks of globules are lofl for want of Thefe difks art 

 proper illumination, but do not change their magnitude on J^j^^J/ 

 that account. The brightnefs of the atmofphere in a fine day light. 

 is fufficient to produce them ; though the illumination of the 

 fun is generally the principal caufe of them. 



(II.) The diameters of fpurious difks are liable to change 

 from various caufes; an alteration in the direction of the illu- 

 mination will make the reflection come from a different part of 

 the globule, which can hardly be expected to be equally po- 

 lifhed in its furface, or of equal convexity every where, being 

 very feldom perfectly fpherical ; but as upon the whole the 

 figure of them is pretty regular, the apparent diameter of the 

 fpurious difks will generally return to. its former iize. 



Globules of mercury were ufed inftead of thofe of filver, and Mercurial glo- 

 with the fame refults. bules - 



The fpurious terfeftrial diffcs were then meafured by com- Mcafurement of 

 paring them with circles on a tablet : and it was found that a ^g ^^ 

 variation in their illumination did not affect their magnitude, diftinguiflied 



It was alfo found that the rays from the central part of the f r <>m real difks 



. . , J , r r . r by ufing firft a 



mirror gave a larger image than thole from its circumference, central, and 



So that when a central aperture of three inches gave an image then an annular 



correfponding with a circle of 0.465 inch, an annular opening g^ e u ,[f a ' rges a ,^ 



from 6.5 to 8.8 inches gave only 0.1 S inch for the image : and thefecond di- 



the experiments werefufftciently varied as to the magnitudes mim 5S c enJ * 



and fituations of the apertures to fhew that this difference did 



cot arife from more or lefs light. 



This property of the mirror ferves admirably to diftinguifh a Trials. 



fpurious difk from a real one ; and proved to be fo on trial with 



terreftriat and celellial objecls. 



Obfervations on ike Nature and Magnitude of Mr, Harding's 

 lately difcovered Star. 



On the day Dr. JH[erfchel received an account of Mr! Obfervations oft 

 Harding's new ftar, which was the 24th of September, he ±c &* Mt J un0i 

 directed his telefcope to the calculated place of the new object, 

 and noted all the fmall ftars within a limited compafs about 

 it. They were then examined with a diftin£l high mag- 

 nifying power ; and fmce no difference in their appearance 



Vox, XII.— Decemeer, 1805. XI was 



