OF AN EIGHTH SATELLITE OF SATURN. 283 



in order from the primary. The first of these was computed by 

 Cassini to have a period of one day and twenty-one hours, and 

 the second of two days and seventeen hours. They are the 

 Tethys and Dione of Sir John Herschel. Cassini employed for 

 their discovery lenses arranged without tubes at enormous focal 

 distances, not less than 155 and 220 Parisian feet. In his me- 

 moir in the Journal des Savans for 1686, he says, — "II nous 

 a ete facile de voir par ces differents sortes de verres ces deux 

 satellites, apres avoir trouve les regies de leur mouvement, qui 

 nous ont fait regarder avec une attention plus particuliere aux 

 lieux ou ils doivent etre." 



These large object-glasses were placed, says Cassini, some- 

 times on the top of the observatory, sometimes on a large pole, 

 and sometimes on a wooden tower transported by order of the 

 king, for this purpose, from Marly to the terrace of the observa- 

 tory. They were afterwards inclosed in tubes. 



The progress of astronomical observation, from this clumsy and 

 helpless machinery to the parallactic movement of Fraunhofer, 

 represents, by a very distinct scale of improvement, the advance- 

 ment of modern science. Although Huyghens had at first been 

 led to adventure the prediction, that his satellite completed the 

 Saturnian group, he lived to see it increased by the four discov- 

 ered by Cassini. In the second book of his K0SM0QES2P0S, 

 addressed to his brother, having alluded to the four satellites 

 discovered by Cassini, he says, — " Imo prseter harum numerum 

 alias quoque vel unam vel plures latere suspicari licet, nee deest 

 ratio. Cum enim inter extremas duas, spatium amplius pateat 

 quam pro distantiis cseterarum, posset hoc insidere sextus satelles, 

 38 



