102 THE HISTORY OF THE TELESCOPE. 



from dispersion, his efforts liad the effect of leading- to much more crit- 

 ical study of the phenomena involved. In this Jolni Dolland, an Eug- 

 lish optician, met with brilliant success. Repeating an experiment of 

 Kewton's with a prism of water opposed by a prism of glass he found 

 that deviation of light could be produced without accompanying dis 

 persion into prismatic colors. More than this, he found that the two 

 varieties of glass, then as now common in England — crown or common 

 window glass, and flint glass, which is characterized by the presence 

 of a greater or less quantity of lead oxide — possessed very different 

 powers in respect to disi>ersi(m; thus, of two prisms of these two vari- 

 eties of glass which would deflect the light by the same angle, that 

 made of flint glass would form a spectrum nearly twice as long as the 

 other; hence, if a prism of crown glass deflecting a transmitted beam 

 of light, say 10 degrees, were combined with one of flint glass which 

 would deflect the beam of light 5 degrees in the opposite direction there 

 would remain a deflection of 5 degrees without division into color. It 

 also follows that a i)ositive lens of crown combined with a negative 

 lens of flint of half the i)ower would yield a colorless image. Such 

 cond)iuations of two different substances are called achromatic systems. 



It is a singular fact, wortli noting in passing, that more than twenty 

 years before Dol land's success, Mr. Chester More Hall had invented 

 and niade achromatic telescopes, but this remained unknown to the 

 world of science until after Dolland's telescopes became tamous. 



For along time this ingenious invention remained fruitless for astro- 

 nomical discovery, (though they were early api)lied to meridian instru- 

 ments,) on account of the impossibility of securing sufficiently large and 

 })erfect pieces of glass, more particularly of flint glass. Not until after 

 the beginning of this century was any real advance in this branch of 

 the arts exhibited. Even then success appeared, not in England or 

 France, where most strenuous efforts had been made to im])rove the 

 quality of optical glass, but in Switzerland. There a humble mechanic, 

 a watchmaker named (ruinaud, spent many years in ettbrts, long un- 

 fruitful, to make large pieces of optical glass. What degree of success 

 he attained therc^ during twenty years of experiment we do not know, 

 though from the fact that during that period good achromatic tele- 

 sco])es of more than 5 inches in diameter were unknown we must con- 

 clude that his success was limited. In 1805 he joined the optical 

 establishment of Fraunhofer and TJtzschneiden in Munich. Here he 

 ren)ained nine years, and with the increased means at his disposal, and 

 the aid of Fraunhofer, he perfected his methods so far that the pro- 

 duction of large disks of homogeneous glass became only a matter of 

 time and cost; that is to say, all of the large pieces of optical glass 

 which have since been produced, whether in Germany, France, or Eng- 

 land, have been made by direct heirs of the i»i'actical secrets of this 

 Swiss watchmaker. 



Fraunhofer was a genius of a liigh order. Although he die(i at the 



