THE HISTORY OF THE TELESCOPE. 101 



when he (liS(H)veiecl by its uieaiis a sixth satellite <>l" Saturn and, less 

 than a month later, a seventh, even closer to the planet and smaller 

 than the sixth. We may regard this a(diievement as markinji the limit 

 of progress in the reflecting telescope, for, although at least one as 

 large is now in use, and one even half as large again has been con- 

 structed, it is inore than doubtful whether they were ever as perfect as 

 Herschers at its best. 



There has been one improvement however in the reflecting telescope 

 since the time of Herschel which ought not to be left unnoticed here, 

 namely, that of replacing the heavy metal mirror by one of glass, made 

 even more highly reflective than the old mirrors by a thin coating of 

 silver deposited by chemical methods upon the jtolished glass. The 

 great advantage of this modern form of reflector lies, not so much in 

 the greater lightness and rigidity of the material as in that the surface 

 when tarnished can be renewed by the simple process of replacing the 

 old silver fllm by a new one; whereas in the metal reflectors a tarnished 

 sui'face required a repetition of the most difficult and critical i)ortion of 

 the whole process of construction. The construction is also so comi>ar- 

 atively simple that an efficient reflector is far less expeusiv^e than are 

 refracting telescopes of like power, so that this may be regarded as 

 particularly the amateur's telescope. On the other hand, such tele- 

 sco})es are, like their ])redecess<)rs. extremely inconstant, and they re- 

 (juire much more careful attention to keep them in working order. It 

 is for these reasons, doubtless, that silver-on-glass reflectors have donc^ 

 so little for the advancement of astronomical discovery. In astroncun- 

 ical pliotograi)hy, however, they promise to do much; and indeed, at 

 the present date by far the best photograi)hs Ave have of any nebuhe 

 have been made by 'Slv. Common's magnificent reflector of 3 feet iu 

 diameter, and by the L*() inch reflector of Mi-. Ifoberts. 



We must go back now to a quarter of a century before Herschel dis- 

 covered the new ))lanct, — to the very year indeed when that great 

 astronomei' first set foot on English soil, — in or<ler to trace tlio history 

 of another form of tele.sco])e whicli has remained unrividlcd for the last 

 half century in the more diflicult lields of astronomical research, and 

 wliich to-day finds its most ])erfe(;t development in the instruments at 

 Mount Hamilton, at Tulkowa, at Vienna, and at Washijigton. 



jS'ewton had declared that, as a result from his exi)eriments, separa- 

 tion of white light into its constituent colors was an inevitable accom- 

 jtaniment of deviation by refraction, and consequently the shortening of 

 the unwieldy retractors was imi)ractical)]e. The (;orrectiH*ss of the ex- 

 Iteriments remained unquestioned for nearly a century; but a famous 

 (Icrnniii mathematician. Fouler, did (piestion his conclusion. His argu- 

 ment was that since the eye does produce colorless images oi' white ob- 

 jects it might be possible by the proper selection of cuives to so combine 

 lenses of glass and of water as to jnoduce a telescope free from the color 

 defect. Although JMilei-'s premise was an error, since the eye is not free 



