474 HISTORY OF PHYSICAL ASTRONOMY. 



of the watch was only one minute five seconds, and the artist received 

 from the nation 5000/. At a later period, 3 at the age of seventy-five 

 years, after a life devoted to this object, having still further satisfied 

 the commissioners, he received, in 1765, 10,000?., at the same time 

 that Euler and the heirs of Mayer received 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 pur- 

 poses ; but the latter could not have been employed at sea without 

 the aid of that invaluable instrument, the Sextant, in which the dis- 

 tance of two objects is observed, by bringing one to coincide apparently 

 with the reflected image of the other. This instrument 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 application 

 of astronomical science which has so materially affected the progress of 

 our knowledge, that it deserves the notice we have bestowed upon it. 



3. Telescopes. — We have spoken of the application of the telescope 

 to astronomical measurements, but not of the improvement of the 

 telescope itself. If we endeavor to augment the optical power of this 

 instrument, we run, according to the path we take, into various in- 

 conveniences ; — distortion, confusion, want of light, or colored images. 

 Distortion and confusion are produced, if we increase the magnifying 

 power, retaining the length and the aperture of the object-glass. If 

 we diminish the aperture we suffer from loss of light. What remains 

 then is to increase the focal leno-th. This was done to an extraordinarv 

 extent, in telescopes constructed in the beginning of the last century. 

 Huyghens, in his first attempts, made them 22 feet long ; 4 afterwards, 

 Campani, by order of Louis the Fourteenth, made them of 86, 100, and 

 136 feet. Huyghens, by new exertions, made a telesrcope 210 feet 

 long. Auzout and Hartsoecker are said to have gone much further, 

 and to have succeeded in making an object-glass of 600 feet focus. 

 But even such telescopes as those of Campani are almost unmanage- 

 able : in that of Huyghens, the object-glass was placed on a pole, and 

 the observer was placed at the focus with an eye-glass. 



The most serious objection to the increase of the aperture of object- 

 glasses, was the coloration of the image produced, in consequence of 

 the unequal refrangibility of differently colored rays. Newton, who 

 discovered the principle of this defect in lenses, had maintained that 



« Mout. iv. 560. 4 Bailly, ii. 253. 



