208 HERSCHEL. 



marked the seventh satellite, then situated at its greatest vrcsteru 

 elongation." 



The 10th of October, 1791, Herschel saw the ring of Saturn and the 

 fourth satellite, looking in at the mirror of his forty-foot telescope, with 

 his naked eye, without any kind of eye-piece. 



Let us acknowledge the true motives that i)revented Herschel from 

 ofteuer using his forty-foot telescope. jSTotwith standing the excellence 

 of the mechanism, the maneuvering of that instrument required the 

 constant aid of two laborers, and that of another person charged 

 with noting the time at the clock. Besides this, during nights when the 

 variation of temperature was considerable, this telescope, on account of 

 its great mass, was always behind the atmosphere in thermometric 

 changes, giving rise to a difference of density in the air within and 

 without the tube very injurious to the distinctness of the images. 



Herscliel found that in England there are not above a hundred hours 

 in a year during which the heavens can be advantageously observed 

 with a telescope of forty feet, furnished with a magnifying power of a 

 thousand. This remark led the celebrated astronomer to the conclusion 

 that to take a complete survey of the heavens with his large instrument, 

 though each successive field should remain only for an instant under 

 inspection, would not require less than eight hundred years. 



He explains very clearly the rare occurrence of the circumstances in 

 which it is possible to make good use of a telescope of forty feet, and 

 of very large aperture. 



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

 air is thicker through w^hich the rays have passed in going to form the 

 image. 



Astronomers expressed extreme surprise when, in 1782, they learned 

 that Herschel had applied linear magnifying i)owers 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 Societj' of London participated in this 

 surprise, and officially" requested Herschel to give publicity to the 

 means he had adopted for using such amounts of magnifying power 

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

 in Volume LXXII 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 Magon, 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 observing brilliant stars; they had not remembered 

 that light reflected by planetary bodies is too feeble to continue distinct 



