Penetration Into Space by Tele/copes. tt 



The 40-feet telefcope was originally intended to have been of the Newtonian conftruc- 

 tion } but, in the yfiar 1787, when I was experimentally aflured of the vaft importance of a 

 power to penetrate into fpace, I laid afide the work of the fmali mirror, which was then 

 in hand, and completed the inftrument in its prefent form. 



" Oft. 10, 1 79 1. I faw the 4th fatellite and the ting of Saturn, in the 40-feet fpecu« 

 " lum, without an eye glafs." 



The magnifying power on that occafion could not exceed 60 or 70 ; but the great pene- 

 trating power made full amends for the lownefs of the former ; notwithftanding the greateft 

 part of it muft have been loft for want of a greater opening of tte iris, which could not 

 take in the whole pencil of rays, for this could not be lefs than 7 or 8 tenths of an inch. 



Among other inftances of the fuperior effefts of penetration into fpace, I (hould mention 

 the difcovery of an additional 6th fatellite of Saturn, on the 28th of Auguft, 1789; and of 

 a 7th, on the nth of September, in the fame year; which were firft pointed out by this 

 inftrument. It is true that both fatellites are within the reach of the ao-feet telefcope : but 

 it fliouldbe remembered, that when an objefl: is once difcovered by a fuperior power, an 

 inferior one will fuffice to fee it afterwards. I need not add, that neither the 7 nor lo-feet 

 telefcopes will reach them 5 their powers, 20 and 29, are not fufficient to pen€trate to fMch 

 diftant objefts, when the brightnefs of them is not more than that of thefe fatellites. It is 

 alfo evident, that the failure in thefe latter inftruments, arifes not from want of magnifying 

 power ; as either of them has much more than fufficient for the purpofe. 



Nov. 5, 1791. I viewed Saturn with the 20 and 40-feet telefcopes. 



«« 20 feet. The 5th fatellite of Saturn is very fmall. The ift, ad, 3d, 4th, 5th, and 

 " the new 6th fatellite, arc in their calculated places." 



" 40-feet. I fee the new 6th fatellite much better with this inftrument than with the 

 *• 20-feet. The 5th is alfo much larger here than in the 20-feet ; in which it was nearly 

 " the famfe fize as a fmall fixed ftar, but here it is confiderably larger than that ftar." 



Here the fuperior penetrating power of the 40-feet telefcope fliewed itfelf on the 6th 

 fatellite of Saturn, which is a very faint obje£l ; as it had alfo a confiderable advantage in 

 magnifying power, the difk of the 5th fatellite appeared larger than in the 20-feet. But 

 the fmall ftar, which may be faid to be beyond the reach of magnifying power, could only 

 profit by the fuperiority of the other power. 



Nov. a I, 179 ». 40-feet refleftor; power 370. 



*' The black divifion upon the ring is as dark as the heavens about Saturn, and of the 

 " fame colour." 



*' The fhadow of the body of Saturn is vifible upon the ring, on the following fide; Its 

 *' colour is very different from that of the dark divifion. The 5th fatellite is lefs than the 

 "3d ; it is even lefs than the 2d." 



20-feet refleftor ; power 3CX). 



C 2 « The 



