278 HISTORY OF PHYSICAL ASTRONOMY. 



at the age of seventy-five years, after a life devoted 

 to this object, having still further satisfied the com- 

 missioners, he received, in 1765, 10,000/., at the 

 same time that Euler and the heirs of Mayer re- 

 ceived each 3000/. for the lunar tables which they 

 had constructed. 



The two methods of finding the longitude, by 

 Chronometers and by Lunar Observations, have 

 solved the problem for all practical purposes; but 

 the latter could not have been employed at sea 

 without the aid of that invaluable instrument, the 

 Sextant, in which the distance of two objects is 

 observed, by bringing one to coincide apparently 

 with the reflected image of the other. This instru- 

 ment was invented by Hadley, in 1731. Though 

 the problem of finding the longitude be, in fact, one 

 of geography rather than astronomy, it is an appli- 

 cation of astronomical science which has so mate- 

 rially affected the progress of our knowledge, that it 

 deserves the notice we have bestowed upon it. 



3. Telescopes. We have spoken of the applica- 

 tion of the telescope to astronomical measurements, 

 but not of the improvement of the telescope itself. 

 If we endeavour to augment the optical power of 

 this instrument, we run, according to the path we 

 take, into various inconveniences ; ^distortion, con- 

 fusion, want of light, or coloured images. Distor- 

 tion and confusion are produced, if we increase the 

 magnifying power, retaining the length and the 

 aperture of the object-glass. If we diminish the 



