the hivention of the Telescope. 15 



remark, but says that "Hortensius wanted more than could be 

 accomplished in his time ; and even now telescopes of a certain 

 size, which show Jupiter's disc well defined, are not of every 

 day's occurrence." The term 'well defined' will of course bear 

 different meanings in different stages of the art : and it is pro- 

 bable that Dr. Moll would be dissatisfied with the performance 

 of a glass, with which Huyghens would have been enraptured. 

 Such expressions are necessarily comparative, and we can only 

 attach a 'well defined' meaning to them by comparing contem- 

 porary statements. The letter of Hortensius to which I alluded 

 contains the following passage {Opere di Gal. torn. ii. p. 4t>6. 

 Ed. Pad.): " De telescopio agere caepimus, comperimusque 

 nulla in Batavia hodie quae tantam praecisionem poUiceri que- 

 ant, quanta ad eas observationes requiritur. Solent enim etiam 

 optimi dikcum Jovis hirsutum offeree et male terminatum, unde 

 Joviales in ejus vicinia non recte conspiciuntur.— Omnes arti- 

 fices rudes experimur, et Dioptricas quam maxime ignaros." 

 Galileo answered in the folUowing terms (p. 474): " Quanto al 

 secondo punto che e del trovarsi Telescopij di maggior effi- 

 cacia di quelli che si fabbricano costi; mi pare d' aver scritto 

 altra voka la facolta di quello che ho adoprato io esser tale, 

 che mostra prlmieramente il disco di Giove non irsuto ma ter~ 

 minatissimo, non meno che V occhio libero scorga il lembo della 

 Luna, e cosi terminati mostra ancora i Satelliti di quello." 

 And in a subsequent letter to Deoclati he says again (p. 472): 

 " Mi vengono ancor domandati dell'istesso Sig. Ortensio i vetri 

 per un Telescopio, i quali sieno di perfezione tale che mostrino 

 ben terminato il disco di Giove, e chiaramente apparenti i 

 quattro suoi Satelliti, effetto che, come egli scrive non se ha da 

 quelli, che si fabbricano in Olanda : se me succedera pronta- 

 nienle il fame provisione, gli inviero a V. S. molt. 111. insieme 

 coUe presente." C. Huyghens had made the same complaint 

 of the inferiority of the Dutch glasses (p. 490): "Del resto i 

 Telescojii che si fanno in queste parti non assicurandoci i quat- 

 tro Satelliti di Giove, de' quali si tratta se non con ceVte scin- 

 tillazioni le quale potrebbero impedire 1' osservazione subite," 

 &c. There is not even the pretext left that Galileo might 

 entertain a better opinion of his glasses than others would 

 have done ; for although Gassendi was dissatisfied with the best 

 glass he could get in Amsterdam, yet on receiving Galileo's 

 present, he wrote (Venturi, vol. ii. p.iil) : "Eximio illo telescopio 

 qno me bcare dignatns es, effigiari lunam procuro suis linea- 

 nientis et coloribus." These extracts show that Dr. Moll has 

 been rather too hasty in advancing that " the assertion of the 

 inferiority of the Dutch telescopes is wholly unsupported by 

 proof." There are a few other trifling oversights in his valu- 

 able communication, which I forbear to notice, fearing lest my 



