ON THE TELESCOPE. 149 



In p. 1 1. Galilceo fays " tubum primo plumbeum mihi pa- Obfervations 



... . J . r . .„ , . ..relating to the 



ravi, in cujusextremitates vitrea duo perlpicdla. . . aptavi. j nve ntion of the 



Again in p. 12. he fays *' dum nulla in tubo adeflent perfpi- telefcope, 

 cilia," the rays pafs in one direction, but *' appofitis perfpi- 

 cillis," they will pafs in another. Hence it appears that Ga- 

 lilaso makes no difference between perfpicilla and vitrea per- 

 fpicilla, but ufes both indifcriminately for the glades of his te- 

 lefcope?;. I am inclined indeed to believe that the original 

 meaning of perfpicillum was like that of conjplcilium a mere 

 lens ; that when lenfes were combined in telefcopes it was 

 ufed both for the glafs and the inftrument, until the word tele- 

 fcopium became common. I am not, however, prepared to 

 bring forward fufficient authorities for this account of the word, 

 and I have not leifure at prefent to fearchfor them. It is fuf- 

 ficient for my purpofe that we are juttified in under/landing the 

 word as meaning a mere lens; for if we confider it as fuch in 



the paflage under confideration all the difficulty vanifhes. 



Kepler complains in this very book decometis* of the weak- 

 nefs of his fight ; he might, therefore, have probably ufed a 

 fimple lens to aflift it. Monfieur Dutens f has clearly proved 

 that the convex lens was known to the ancients, and at the 

 fame time J that Borelli calls Zachary Janfens, '* confpicilio- 

 rum artifex peritiffimus," he fpeaks of his concave as well as 

 convex glafies. 



There is one circumftance attending the obfervation itfelf, 

 which feems to give fome fupport to my interpretation, which 

 is, that it was not made at a time when Kepler was carefully 

 examining the heavens for any new phenomena; but in a walk 

 which he accidentally took with his friend. Now he may eafily 

 be fuppofed to have carried about with him a fimple lens to 

 correct any fault of his eyes, but it is fcarcely probable that he 

 would have had with him a telefcope, made, poffibly like Ga- 

 lileo's, of a long leaden pipe. I fcarcely need add, that 

 whatever clears and firengthens the fight will diminifii the ap- 

 parent diameter of a fixed fiar, and confequently increafe the 

 relative magnitude and light of the comet. 



• P. 30. Inftrumenta Tychonica, &c. &c. et oculos vegetos re- 

 quirunt, quse omnia mihi defuerunt, 1607, and in p. 48. Debili 

 fum vim, 1618. 



f Decouvcrtes des anciens. Partie 3, chap. 10. §. y?S. X P«25. 



I am 



