8 TRANSIT OF MERCURY. 



II. 



I 



{Concluded from Page 304. Vol. V ) 



Ohfervations of the Tranfit of Mercury over the Difk of the Sun ; 

 to which is added, an Invefti^ution of the Caitfes tchich often 

 prevent the proper A6iion of Mirrors. By William Her- 

 scHEL, LL.D. F.R.S. 



J^ercury neatly vV ITU a 1 0-feet refleaor, and magnifying power of 1 30, I 

 faw the corrugations of the luminous folar furface, up to the 

 very edge of the whole periphery of the difk of Mercury. 



10 27'. When the planet was fufficiently advanced towards 

 the largefi: opening of the northern zone, I compared ihe in- 

 tenfity of the blacknefs of the two objeds ; and found the difk 

 of Mercury confiderably darker, and of a more uniform black 

 tint, than the area of the large opening. 



lO*" 32'. The preceding limb of Mercury cuts the luminous 

 folar clouds with the moft perfe6l fliarpnefs ; whereas, in the 

 great opening, the defcending parapet, down the preceding 

 iide, was plainly vifible. 



It fliould be remarked, that the inftrument here applied to 



the fun, with the moderate power of 130, is the fame 10-feet 



refledor which, in fine nights, when direfled to very minute 



double liars, will ftiovv them diftindly with a magnifier of 



1000. 



Great magni. Having often attempted to ufe high magnifiers in viewing 



fying power not the Am, I wiflicd to make another trial ; though pretty well 



fua, ^ afTured I fhould not fucceed, for reafons which will appear 



hereafter. 



With two fmall double convex lenfes, both made of dark 

 green glafs, and one of them having the fide which is nearefl 

 the eye thinly fmoked, in order to take off fome light, I viewed 

 the fun. Their magnifying power was about 300; and I faw 

 Mercury very well defined; but that complete diftin<5tnefs, 

 which enables us to judge with confidence of the condition 

 of the objed in view, was wanting. 



With a fingle eye-glafs, fmoked on the fide towards the eye, 

 and magnifying 460 times, I alfo faw Mercury pretty well 

 defined ; but here the fun appeared ruddy, and no very mi- 

 pute objeds could be perceived. 



