HIS IMPROVEMENTS IN THE TELESCOPE. 275 



Those telescopes, in which the observer, placed at the 

 anterior extremity of the tube, looks direct into the tube 

 and turns his back to the objects, were called by Herschel 

 front view telescopes. In vol. Ixxvi. of the Philosophi- 

 cal Transactions he says, that the idea of this con- 

 struction occurred to him in 1776, and that he then 

 applied it unsuccessfully to a ten-foot telescope ; that 

 during the year 1784, he again made a fruitless trial of 

 it in a twenty-foot telescope. Yet I find that on the 7th 

 of September 1784, he recurred to a front view in ob- 

 serving some nebulas and groups of stars. However dis- 

 cordant these dates may be, we cannot without injustice 

 neglect to remark, that a front view telescope was already 

 described in 1732, in volume vi. of the collection entitled 

 Machines and Inventions approved by the Academy of 

 Sciences. The author of this innovation is Jaques Le- 

 maire, who has been unduly confounded with the English 

 Jesuit, Christopher Maire, assistant to Boscovitch, in 

 measuring the meridian comprised between Rome and 

 Rimini. Jaques Lemaire having only telescopes of mod- 

 erate dimensions in view, was obliged, in order not to 

 sacrifice any of the light, to place the great mirror so 

 obliquely, that the image formed by its surface should fall 

 entirely outside the tube of the instrument. So great a 

 degree of inclination would certainly deform the objects. 

 The front view construction is admissible only in very 

 large telescopes. 



I find in the Transactions for 1803, that in solar ob- 

 servations, Herschel sometimes employed telescopes, the 

 great mirror of which was made of glass. It was a 

 telescope of this sort that he used for observing the 

 transit of Mercury on the 9th of November, 1802. It 

 was seven English feet long, and six inches and three 

 tenths in diameter. 



