Chap; J.] Ga!iko,Kepkr>&f. xj 



into common ufe. The inventor of the tele'fcope is 

 not certainly known. The moft probable account is, 

 that one Zacharias Janfen, a fpectacle maker of Mid- 

 dleburgh, trying the effect of a concave and convex 

 glafs united, found that, placed at a certain diftance 

 from each other, they had the. property of bringing 

 diftant objects apparently nearer to the eye *. Tele- 

 fcopes were greatly improved by Galileo, who made 

 one to magnify thirty-three times, and with this he^ 

 made all his wonderful aftronomical difcoveries. 



The rationale of telefcopes was, however, not ex- 

 plained till Kepler, who defcribed the nature and the 

 degree of refraction, when light parted through denfer 

 or rarer mediums, the furfaces of which are convex or 

 concave, namely, that it correfponds to the diameter 

 of the circle of which the convexity or concavity are 

 portions of arches. He fuggefted fome improvements 

 in the construction of telefcopes, which, however, 

 were left to others to put in practice. 



To the Janfens we are alfo indebted for the difcovery 

 of the microfcopej an inftrument depending upon 

 exactly the fame principles as the former. In fact, it 

 is not improbable that the double lens was firft applied 

 to the obfervation of near but minute objects, and 

 afterwards, on the fame principles, to objects which 

 appeared minute on account of their diftance. 



Much attention was given by Kepler to the invef- 

 tigation of the law of refraction ; but he was. able to 



* An account which is very commonly received is, that fome 

 f his children playing in his fhop with fpe&acle glafles, perceiv- 

 ed that when they held two of thefe glafles between their fingers, 

 at a certain diftance from each other, the dial of the clock ap- 

 peared greatly magnified, but in an inverted pofition. From this 

 their father took the idea of adjufting two of thefe glailes on a 

 board, fo as to move them at pleafure. 



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