AIDS OF THE NEWTONIAN PERIOD. 475 



Jie evil was irremediable, and that a compound lens could no more 

 refract without producing color, than a single lens could. Eulcr and 

 Klingenstierna doubted the exactness of Newton's proposition ; and, 

 in 1755, Dollond disproved it by experiment. This discovery pointed 

 out a method of making object-glasses which should give no color ; 

 which should be achromatic. For this purpose Dollond fabricated 

 various kinds of glass (flint and crown glass) ; and Clairaut and 

 D'Alembert calculated formula?. Dollond and his son 5 succeeded 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 discoveries 

 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 

 compendious size of the new instruments ; for, in increasing the di- 

 mensions, the optician was stopped by the impossibility of obtaining 

 lenses of flint-glass of very large dimensions. And this branch of art 

 remained long stationary ; but, after a time, its epoch of advance again 

 arrived. In the present century, Fraunhofer, at Munich, with the help 

 of Guiuand and the pecuniary support of Utzschneider, succeeded in 

 forming lenses of flint-glass of a magnitude 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. 



[2d Ed.] [Joseph Fraunhofer was born March 6, 1787, at Strau- 

 bing in Bavaria, the son of a poor glazier. He was in his earlier 

 years employed in his father's trade, so that he was not able to attend 

 school, and remained ignorant of writing and arithmetic till his four- 

 teenth year. At a later period he was assisted by Utzschneider, and 

 tried rapidly to recover his lost ground. In the year 1806 he entered 

 the establishment of Utzschneider as an optician. In this establish- 

 ment (transferred from Benedictbeuern to Munich in 1819) he soon 

 2:une to be the greatest Optician of Germany. His excellent tele- 

 scopes and microscopes are known throughout Europe. His greatest 

 telescope, that in the Observatory at Dorpat, has an object-glass of 9 

 inches diameter, and a focal length of 13J feet. His written produc- 

 tions are to be found in the Memoirs of the Bavarian Academy, in 

 Gilbert's Annalen der Physik, and in Schumacher's AstronomiscJu 

 NacJirichten. He died the 7th of June, 1826.] 



5 Bailly, iii. 113. 



