128 LIGHT SCIENCE FOR LEISURE HOURS. 



to be on a supposition of its not being a fixed star, 

 while the diameters of the stars to which I compared 

 it were not increased in the same ratio. Moreover, the 

 comet being magnified much beyond what its light 

 would admit of, appeared hazy and ill-defined with 

 these great powers, while the stars presented that lustre 

 and distinctness which from many thousand observa- 

 tions I knew they would retain. The sequel has shown 

 that my surmises were well-founded.' 



There are three points to be specially noted in this 

 account. First, the astronomer was engaged in a 

 process of systematic survey of the celestial depths so 

 that the discovery of the new orb cannot be properly 

 regarded as accidental, although Herschel was not at 

 the time on the look-out for as yet unknown planets. 

 Secondly, the instruments he was employing were of 

 his own construction and device, and probably no 

 other in existence in his day would have led him to 

 the discovery that the strange orb was not a fixed star. 

 And thirdly, without the experience he had acquired 

 in the study of the heavens he would not have been 

 able to apply the test which, as we have seen, he found 

 so decisive. The fact that the stars are not magnified 

 by increased telescopic power to the same extent as 

 planets or comets, is, as Professor Pritchard has justly 

 remarked, c an important result of the undulatory theory 

 of light, and was unsuspected in Sir William Herschel's 

 day.' So that whether we consider the work Herschel 

 was engaged upon, the instruments he used, or the ex- 

 perience he had acquired, we recognise the fact that he 



