164 ANNUAL OF SCIENTIFIC DISCOVERY. 



laston noticed that when he allowed the sunlight to fall through a 

 narrow slit upon the prism, a number of dark lines, cutting up 

 the colored portions of the spectrum, made their appearance. These 

 dark lines, or spaces, of which Wbllaston counted only seven, indicate 

 the absence of certain distinct kinds of rays in the sunlight ; they are, 

 as it were, shadows on the bright background. 



It is, however, to the celebrated German optician Fraunhofer that 

 we owe the first accurate examination of these singular lines. By a 

 great improvement in the optical arrangements employed, Fraunhofer, 

 rediscovering these lines, was able to detect a far larger number of 

 them in the solar spectrum than had been observed by Wollaston. He 

 counted no less than five hundred and ninety of these dark lines, 

 stretching, throughout the length of the spectrum, from red to violet, 

 and in the year 1815 drew a very beautiful map of them, some of the 

 most important of which he designated by the letters of the alphabet. 

 Fraunhofer carefully measured the relative distances between these 

 lines, and found that they did not vary in sunlight examined at differ- 

 ent times. He also saw these same dark fixed lines in reflected as 

 well as in direct solar light ; for on looking at the spectrum of moon- 

 light and of Venus-light the same lines appeared quite unaltered in 

 position. But he found that the light of the fixed stars was not of 

 the same kind as direct or reflected sunlight, as the spectra of the 

 starlight contained dark lines entirely different from those which are 

 invariably seen in the solar spectrum. From these observations 

 Fraunhofer, so early as 1815, drew the important conclusion that 

 these lines, let them be what they may, must in some way or other 

 have their origin in the sun. The explanation of the production of 

 these lines was reserved for a subsequent time ; but Fraunhofer 

 opened the inquiry, and all his conclusions have been borne out by 

 recent and more elaborate investigations. 



Since the time of Fraunhofer our knowledge of the constitution 

 of the solar spectrum has largely increased. Professor Stokes, in his 

 beautiful researches on fluorescence, has shown that similar dark 

 lines exist in that part of the spectrum extending beyond the violet, 

 which require special arrangements to become visible to our eyes ; 

 and Sir David Brewster and Dr. Gladstone have mapped with great 

 care about two thousand lines in the portion of the spectrum from 

 red to violet. 



But it is to KirchhofF, the Professor of Physics in the University of 

 Heidelberg, that we are indebted for by far the best and most accu- 

 rate observations of these phenomena. In place of using one prism, 

 as Fraunhofer did, Kirchhoff employed four prisms of most perfect 

 workmanship, and thus enjoyed the advantage of a far greater dis- 

 persion, or spreading out, of the different rays than the Munich opti- 

 cian had obtained. The lines were observed through a telescope 

 having a magnifying power of forty, and when the whole apparatus 

 was adjusted with all the accuracy and delicacy which the perfection 

 of optical instruments now renders possible, Kirchhoff saw the solar 

 spectrum with a degree of minute distinctness such as had never 

 before been attained; and of the beauty and magnificence of the 

 sight thus presented those only who have been eye-witnesses can 

 form anv idea. 



