280 HISTORY OF PHYSICAL ASTRONOMY. 



(flint and crown glass;) and Clairaut and D'Alem- 

 bert calculated formulae. Dollond and his son 5 suc- 

 ceeded in constructing telescopes of three feet long 

 (with a triple object-glass) which produced an effect 

 as great as those of forty-five feet on the ancient 

 principles. At first it was conceived that these dis- 

 coveries opened the way to a vast extension of the 

 astronomer's power of vision ; but it was found that 

 the most material improvement was the compen- 

 dious size of the new instruments ; for, in increasing 

 the dimensions, the optician was stopped by the 

 impossibility of obtaining lenses of flint-glass of 

 very large dimensions. And this branch of art re- 

 mained long stationary ; but, after a time, its epoch 

 of advance again arrived. In the present century, 

 Fraunhofer, at Munich, with the help of Guinaud 

 and the pecuniary support of Uzschneider, suc- 

 ceeded in forming lenses of flint-glass of a magni- 

 tude till then unheard of. Achromatic object- 

 glasses, of a foot in diameter, and twenty feet focal 

 length, are now no longer impossible ; although in 

 such attempts the artist cannot reckon on certain 

 success (w). 



Such telescopes might be expected to add some- 

 thing to our knowledge of the heavens, if they had 

 not been anticipated by reflectors of an equal or 

 greater scale. James Gregory had invented, and 

 Newton had more efficaciously introduced, reflect- 

 ing telescopes. But these were not used with any 

 5 Bailly, iii. 118. 



