THE IllSTtrnV OF THE TELESCOPE. 90 



cUite.s of kSaturu, however, as maiUiii-i tlie \ ery taitlicst reaeli of tlie 

 old form of telescope; a century was to elapse and an entirely new 

 form of telescope was to be developed before another considerable ad- 

 dition to onr knowledge' of the aspect of the heavenly- bodies was to be 

 iiiadt\ It is true larger telescopes were made, and ITuygheus invented 

 a means by wliich they could be used without tubes, but notwithstand- 

 ing this improvement they proved so cumbers(une as to be impracticable. 



The older o[)ticians had tbund that if they attempted to increase the 

 diameter of a teleseojje they were obliged to increase its length in a 

 much more rapi<l ratio to secure distinct vision. The reason of this 

 was not clearly understood, but it was sui)posed to be owing to the fact 

 that a wave front, changed in curvature by passing througli a s])herical 

 surface, is no longer strictly spherical. This deviation in shai)e of tlu; 

 refracted wave from a true sphere is called spherical aberration. When 

 the refracting surfaces are large and of considerable curvature this 

 soon becomes very serious, but by using small curvature, which, in a 

 telescope, obviously conesponds to great length, theefl'eets of the error 

 can l)e made insensible. Xewton's discovery of the com])osile, nature 

 of light and of the phenomenon of dispersion enabled him to explain 

 the true cause of indistinctness in short telescoi)es: mimely, that the 

 refraction by the objective varies for different colors; conseiiuently, if 

 the ocular is placed for one particular color, it will not be in the light 

 position for any of the others, when(;e the image of a star or ])laiiet 

 will stem to be suirounded by a fi'inge of colored light. Newton found 

 this source of indistinctness in the image, wliich is now known as 

 cliromati(; aberration, many hnndred times as serious as the s})her 

 ica I aberrations. As he was i)ersuaded by his ex])eriments that this 

 obstacle to further improvement in the refracting telescope was in- 

 snperable, he turned his attention to a form of teleseojn' m hich had 

 been suggested a number of years earlier in which (he image was to 

 be formed by retlection from a concave mirror, and constructed a small 

 one with his own hands which is still in the possession of the b'oyal 

 Society. This little instrument seems to have been ol" about the same 

 ]»ower as Galileo's instrument with which he discovered the satellites 

 of Jupiter, but is was hardly more than (J inches in length. 



Since that time the retlecting telescope has had a remarkable history 

 of develo])menl in the hands of a number of most skillful mechani- 

 cians, who liave also for the most part beeu distinguished by their dis- 

 coveries in i»liysical astronomy; we may therelbre advantageously 

 depart from the chronological treatment and follow the history of this 

 type of instrument. This c(mrse is the more natural because we may 

 probably regard the snpremacy of the relleetor (undisputed a century 

 ago) as passed away tore\'er. 



Even after Newton's invention was made ])ul»lic, little was done 

 towards the improvement of telescopes Ibi- half a century, until Hadley 

 presented a relleetor of his own construction to the iioyal Society iu 



