90 LIGHT SCIENCE FOR LEISURE HOURS. 



points, small as the point of a needle, and close as the 

 particles of a handful of sand ; so that it needs but the 

 smallest unsteadiness in the air, or imperfection in the 

 shape of the reflecting surface, to scatter the light of 

 each point, to merge them into each other, and present 

 them as one mass. 



Before long Lord Eosse, after regrinding the great 

 mirror, obtained better views of the mysterious nebula. 

 Even now, however, he could use but half the power of 

 the telescope, yet at length the doubts of astronomers 

 as to the resolvability of the nebula were removed. 

 We could plainly see, he wrote to Professor Nichol, 

 that all about the trapezium was a mass of stars, the 

 rest of the nebula also abounding with stars, and ex 

 hibiting the characteristics of resolvability strongly 

 marked. These views were abundantly confirmed by 

 subsequent observations with the great mirror. 



It will surprise many to learn that even Lord Eosse s 

 great reflector is surpassed in certain respects by some of 

 the exquisite refractors now constructed by opticians. 

 As a light-gatherer the mirror is, of course, unapproach 

 able by refractors. Even if it were possible to construct 

 an achromatic lens six feet in diameter, yet, to prevent 

 flexure, a thickness would have to be given to the glass 

 which would render it almost impervious to light and 

 therefore utterly useless. But refractors have a power 

 of definition not possessed by large reflectors. With a 

 refractor eight inches only in aperture, for instance, 

 Mr. Dawes has obtained better views of the planets (and 

 specially of Mars), than Lord Eosse s six-feet speculum 



