HIS FORTY-FOOT TELESCOPE. 277 



lite, the Philosophical Transactions for 1790, p. 10.) In 

 that same volume of 1790, p. 11, I find: "The great 

 light of my forty-foot telescope was then so useful, that 

 on the 17th of September 1789, I remarked the seventh 

 satellite, then situated at its greatest western 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 sort of eye-piece. 



Let us acknowledge the true motives that prevented 

 Herschel from often er using his telescope of forty feet. 

 Notwithstanding the excellence of the mechanism, the 

 manosuvring of that instrument required the constant aid 

 of two labourers, and that of another person charged with 

 noting the time at the clock. During some nights when 

 the variation of temperature was considerable, this tele- 

 scope, on account of its great mass, was always behind- 

 hand with the atmosphere in thermometric changes, 

 which was very injurious to the distinctness of the 



images. 



Herschel 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 re- 

 main only for an instant under inspection, would not 

 require less than eight hundred years. 



Herschel explains in a very natural way the rare oc- 

 currence of the circumstances in which it is possible to 

 make good use of a telescope of forty feet, and of very 

 large aperture. 



