284 HERSCHEL. 



Under equality of subtended angle, then, the telescopic 

 vision with strong magnifying powers showed itself supe- 

 rior to the naked eye vision. This result is not unim- 

 portant. 



If we take notice of the magnifying powers used by 

 Herschel in these laborious researches, powers that 

 often exceeded five hundred times, it will appear to be 

 established that the telescopes possessed by modern as- 

 tronomers, may serve to verify the round form of distant 

 objects, the form of celestial bodies even when the diame- 

 ters of those bodies do not subtend naturally (to the naked 

 eye), angles of above three tenths of a second : and 500, 

 multiplied by three tenths of a second, give 2' 30." 



Refracting telescopes were still ill understood instru- 

 ments, the result of chance, devoid of certain theory, 

 when they already served to reveal brilliant astronomical 

 phenomena. Their theory, in as far as it depended on 

 geometry and optics, made rapid progress. These two 

 early phases of the problem leave but little more to be 

 wished for ; it is not so with a third phase, hitherto a good 

 deal neglected, connected with physiology, and with the 

 action of light on the nervous system. Therefore, we 

 should search in vain in old treatises on optics and on as- 

 tronomy, for a strict and complete discussion on the compaiv 

 ative effect that the size and intensity of the images, that 

 the magnifying power and the aperture of a telescope 

 may have, by night and by day, on the visibility of the 

 faintest stars. This lucana Herschel tried to fill up in 

 1799 ; such was the aim of the memoir entitled, On the 

 space-penetrating Power of Telescopes. 



This memoir contains excellent things ; still, it is far 

 from exhausting the subject. The author, for instance, 

 entirely overlooks the observations made by day. I also 



