SKETCH OF DR. JOSEPH FRAUNHOFER. 741 



at Munich. Four years before his death, this Academy appointed him 

 keeper of its Museum of Physics. lie had the Order of Civil Merit 

 conferred on him by the King of Bavaria, and received the Order of 

 Danebrog from the King of Denmark. 



It was as a scientific optician, not only thoroughly familiar with 

 the theory of the subject, but skillful in the use of instruments, and 

 an accurate and painstaking observer, that Fraunhofer entered upon 

 the exploration of a new phenomenon in the solar spectrum. 



It is well known that we are indebted to Sir Isaac Newton for the 

 capital experiment of the decomposition of white light into its con- 

 stituent color-rays. He passed the beam from an opening in a shutter 

 through a glass prism in a darkened room, and got the image of colors 

 in the order of their refrangibility, forming what is familiarly known 

 as the solar spectrum. But this spectrum is not pure. He used the 

 light from a round hole in the shutter, and the ray, when passing 

 through the prism, gave a series of overlapping images of the aperture, 

 by which the colors of the spectrum were somewhat mixed, and, in 

 consequence of this, there was a peculiar class of effects which he did 

 not recognize. His experiment was made in the year 1675, exactly 

 200 years ago, and for 127 years his method of forming the spectrum 

 was folloAved, and no step was taken toward the discovery of the phe- 

 nomena now to be considered. But, in 1802, Dr. Wollaston, an Eng- 

 lishman, examined the spectrum formed by a narrow opening or slit, 

 and found that instead of being so pure as had always been supposed, 

 it was crossed in various places by fine dark lines. The observation 

 attracted no attention at the time, and was not followed up by himself 

 or others. 



These dark lines were afterward rediscovered by Fraunhofer, who 

 became so much interested in them that he made them the subject of 

 careful investigation, and his results were so accurate and complete 

 as to have been universally accepted when published in 1814, and the 

 lines from that time have gone under the name of " Fraunhofer's lines." 

 By means of a telescope he observed the spectrum formed by a fine 

 slit, and found that it was crowded with these fine dark lines ; that 

 they varied somewhat in thickness, and were distributed in unecpial 

 groups throughout the spectral space. He counted 590, from the red 

 to the violet, and made an accurate map of them, as represented in 

 the preceding figure, designating the most important by the letters of 

 the alphabet, large and small, which are still constantly used in the 

 investigations of spectrum analysis. 



To the question, What are these dark lines ? no clear answer could 

 be given. Science was not as yet prepared to offer an explanation of 

 their cause. Yet Fraunhofer's mind was not idle in regard to this 

 point, and he speculated with great sagacity in the right direction. 

 Optically, or with reference to figure, the dark lines are simply images 

 of the slit. There are of course no such lines in sunlight, but there is 



