278 HERSCHEL. 



A telescope does not magnify real objects only, but 

 magnifies also the apparent irregularities arising from 

 atmospheric refractions ; now, all other things being equal, 

 these irregularities of refraction must be so much the 

 stronger, so much the more frequent, as the stratum of 

 air is thicker through which the rays have passed to go 

 and form the image. 



Astronomers experienced extreme surprise, when in 

 1782, they learned that Herschel had applied linear 

 magnifying powers of a thousand, of twelve hundred, of 

 two thousand two hundred, of two thousand six hundred, 

 and even of six thousand times, to a reflecting telescope 

 of seven feet in length. The Royal Society of London 

 experienced this surprise, and officially requested Her- 

 schel to give publicity to the means he had adopted for 

 ascertaining such amounts of magnifying power in his 

 telescopes. Such was the object of a memoir that he in- 

 serted in vol. Ixxii. of the Philosophical Transactions ; 

 and it dissipated all doubts. No one will be surprised 

 that magnifying powers, which it would seem ought to 

 have shown the Lunar mountains, as the chain of Mont 

 Blanc is seen from Macon, from Lyons, and even from 

 Geneva, were not easily believed in. They did not 

 know that Herschel had never used magnifying powers 

 of three thousand, and six thousand times, except in ob- 

 serving brilliant stars ; they had not remembered that 

 light reflected by planetary bodies, is too feeble to con- 

 tinue distinct under the same degree of magnifying power 

 as the actual light of the fixed stars does. 



Opticians had given up, more from theory than from 

 careful experiments, attempting high magnifying powers, 

 even for reflecting telescopes. They thought that the 

 image of a small circle cannot be distinct, cannot be 



