os Jeo! oe ee eS 
Ps a ee 
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 
Sront view telescopes. In vol. lxxvi. 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 nebule 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 1782, 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 Meréury on the 9th of November, 1802. It 
was seven English feet long, and six inches and three 
tenths in diameter. 
