200 GREAT MEN OF SCIENCE 



that 'these Hnes lie in the nature of the sun's light, and do not 

 arise by diffraction, self-deception, etc.'^ 



He always actually obtained them under all conditions in 

 the same manner as soon as a pure spectrum of sunlight or 

 daylight was formed. He also sought them in the light 

 from Venus, and found them there. Fixed stars, some 

 bright ones which he investigated, showed lines differently 

 grouped. Fraunhofer was not yet able to realise that this 

 opened the way to chemical analysis of the atmospheres of the 

 sun and fixed stars, to which the combined efforts of Bunsen 

 and Kirchhoff led forty-five years later. Nevertheless the 

 lines of the sun's light, peculiar to it, remained a starting 

 point for future investigations. Fraunhofer avoided in his 

 extremely modest, but, as regards everything actually ascer- 

 tainable, very penetrating manner, every discussion of mere 

 supposition. Instead, he gives us an exact drawing of these 

 dark lines of the sun's spectrum, a drawing which remained 

 unsurpassed in its perfection until the time of Kirchhoff. He 

 also immediately used the lines - which he designated by 

 letters from A in the red to H in the violet, as we do to-day - 

 as fixed points in the sun's spectrum, which enabled him 

 always to select quite exactly defined kinds of light, which 

 had not been possible hitherto, when only the colour could be 

 named. 



This was of great importance when it was necessary to 

 measure the refractive index of a variety of glass for lenses 

 for different colours, an operation indispensable for the con- 

 struction of good achromatic (colour-free) telescope lenses. 

 More than sixty years ago, the construction of achromatic 

 lenses from two different kinds of glass had already been begun, 

 but success was uncertain, if the refractive index of the two 

 glasses for different colours did not fulfil certain conditions. 



^ This statement forms the difference between Fraunhofer's discovery 

 and a few previous observations of such lines in the form of chance phe- 

 nomena varying with the apparatus, which observations, being indefinite 

 in character, had been forgotten. 



