Mr. Drinkvvater on the Invention of the Telescope. 9 



tlie same ray in both observations as the mean one, the mean 

 extraordinary index will be as follows : 



Mean Extr. Index. Dift". 



Rudberg 1-48971 p.. 



Mains 1-48330 



Dr. Wollaston gives the two indices according to his mea- 

 surements, as r657 and 1*488, which are nearer those ofM. 

 Rudberg; but as Dr. Young has assured us that the measures 

 taken by Dr. Wollaston are appropriate to the extreme red 

 rays, they throw no light upon the cause of the discrepancy 

 under our consideration. The discrepancies, indeed, become 

 more alarming, and will stand thus, by calling B the ex- 

 treme red ray, and calculating its index from Malus's index 

 for the middle green. 



Red Ray B. 

 Ordinary. Extraordinary. 

 Wollaston ... 1-65700 1-48800 



Rudberg 1-65308 1-48391 



Mains 1-64152 1*47750 



Hence it appears that Wollaston's measures are greater than 

 those of Rudberg, and still more remote from those of Malus. 



As it is impossible to suppose for a moment that either 

 Malus or Rudberg could have committed errors of observa- 

 tion capable of reconciling these results, vve are forced to con- 

 clude, that the experiments of each were made with crystals 

 of calcareous spar which had different degrees of refractive 

 power, — a conclusion which deprives us of the hope of ob- 

 taining invariable physical data for different minerals. Such 

 a result, indeed, might have been expected from the variety 

 of specific gravities which different specimens of calcareous 

 spar exhibit; and all that science can hope to accomplish on 

 this subject, is to define the general limits within which these 

 variations are confined. 



III. Observations respecting the Invention of the Telescope. 

 By J. E. Drinkwateu, Esq.* 



AVERY interesting article on the invention of telescopes 

 is printed in the second and third Numbers of the Journal 

 of the Royal Institution, in which it is clearly proved that 

 John Lippershey, a spectacle-maker of Middleburg, possessed 

 the invention on the 2nd of October, 1608: that Jacob Adri- 



* Coninuuiicated by the Autlior: — In the first series of Phil. \fag. vol.xviii. 



. p, 245, and vol. xix. p. 66, will he found two letterb, with extracts from 



old English books, from whicli tlie writer infers that the tele>;cope was 



known in En/^land long before the jierioil of its reputed invention. — EuiT. 



Third Series. Vol. 1. No. 1. Jidy 1832. C 



