256 DISCOURSE ON THE STUDY 



plete and satisfactory as that of hearing by the propa- 

 gation of motion through the air. The camera ob- 

 scura, invented by Baptista Porta in 1560, gave the 

 first idea how the actual images of external objects 

 might be conveyed into the eye, but it was not till 

 after a considerable interval that Kepler, the im- 

 mortal discoverer of those great laws which regu- 

 late the periods and motions of the planets, pointed 

 out distinctly the offices performed by the several 

 parts of the eye in the act of vision. From this to 

 the invention of the telescope and microscope 

 there would seem but a small step, but it is to acci- 

 dent rather than design that it is due ; and its re- 

 invention by Galileo, on a mere description of its 

 effects, may serve, among a thousand similar in- 

 stances, to show that inestimable practical appli- 

 cations lie open to us, if we can only once bring 

 ourselves to conceive their possibility, a lesson 

 which the invention of the achromatic telescope it- 

 self, as we have above related it, not less strongly 

 exemplifies. , 



(283.) The little instrument with which Galileo's 

 splendid discoveries were made was hardly supe- 

 rior in power to an ordinary finder of the present day; 

 but it was rapidly improved on, and in the hands of 

 Huyghens attained to gigantic dimensions and very 

 great power. It was to obviate the necessity of the 

 enormous length required for these telescopes, and 

 yet secure the same power, that Gregory and Newton 

 devised the reflecting telescope, which has since 

 oecome a much more powerful instrument than its 

 original inventors probably ever contemplated. 



(284.) The telescope, as it exists at present, with 



