The Evolution of the Sciences 



The invention of the reflecting telescope 

 marked a further advance; in this invention, 

 successively improved by Gregory, Newton, and 

 especially by Sir William Herschel, one of the 

 lenses is replaced by a concave mirror. No one 

 has known more thoroughly, or described more 

 minutely, the heavenly paths than the celebrated 

 astronomer of Slough. No one has had at his 

 disposal more powerful instruments. Herschel 

 found in England the necessary support to enable 

 him to construct instruments whose magnifying 

 power has never been surpassed. Such was the 

 famous telescope, forty feet long and six feet in 

 diameter, completed in 1789. However, this 

 instrument showed Herschel that the most 

 powerful telescopes are not always the best 

 for purposes of observation; their enormous 

 size makes them difficult to handle and distorts 

 the image on the mirror; the draught in the tube, 

 the ceaseless and unavoidable agitation of the 

 air, the expansions caused by the heat of the 

 sun all disturb the normal course of the rays. 

 For this reason modern observers prefer to use 

 instruments more modest in appearance but 

 capable of equally good results. 



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