?6 Dr. Herschel on the Power of 



telescope; penetrating power ■ 4X4 ° = lgi, 6g; magnify- 



z 



mg power 370. 



" A beautiful cluster of stars. I counted about 200 of them. 

 " The middle of it is so compressed that it is impossible to dis- 

 " tinguish the stars." 



Here it appears, that the superior penetrating power of the 40- 

 feet telescope enabled me even to count the stars of this nebula. 

 It is also to be noticed, that the object did not strike me as 

 uncommonly beautiful ; because, with much more than double 

 the penetrating, and also more than double the magnifying 

 power, the stars could not appear so compressed and small as 

 in the 20-feet instrument: this, very naturally, must give it 

 more the resemblance of a coarser cluster of stars, such as I had 

 been in the habit of seeing frequently. 



The 40-feet telescope was originally intended to have been 

 of the Newtonian construction ; but, in the year 1787, when 

 I was experimentally assured of the vast importance of a power 

 to penetrate into space, I laid aside the work of the small mirror, 

 which was then in hand, and completed the instrument in its 

 present form. 



" Oct. 10, 1791. I saw the 4th satellite and the ring of 

 " Saturn, in the 40-feet speculum, without an eye glass." 



The magnifying power on that occasion could not exceed 60 

 or 70 ; but the great penetrating power made full amends for 

 the lowness of the former; notwithstanding the greatest part 

 of it must have been lost for want of a greater opening of the 

 iris, which could not take in the whole pencil of rays, for this 

 could not be less than 7 or 8 tenths of an inch. 



