FRAUNHOFER'S LINES. 505 



yellowish, tinge. But if solar light is more closely 

 observed by means of the spectroscope, a very large 

 number of fine black lines is seen, of different degrees 

 of breadth and shade, which are always present and 

 always occupy exactly the same relative positions in the 

 solar spectrum ; they are called Fraunhofer' 's lines after 

 Fraunhofer of Munich, who was the first to study these 

 lines with much minuteness. The spectroscope used in 

 the preceding experiments exhibits prominently three 

 such lines, in the yellow, green, and blue, one in each 

 colour, as represented in the solar spectrum of fig. L; but 

 a careful examination shows the whole of the spectrum to 

 be striped, especially in the green and blue portion. The 

 greater the dispersive and optical power of a spectro- 

 scope, the greater is the number of dark lines seen in 

 the solar spectrum ; in the largest spectroscopes the 

 number amounts to several thousand. The cause of 

 the striped appearance of the spectrum seen by our 

 bisulphide of carbon prism is, that the lines are too near 

 to one another to be seen single ; only the three lines 

 above mentioned, which are the most marked, appear 

 really as distinct dark lines, while of the others, whole 

 groups, by merging into one another, produce the striped 

 shading of the spectrum as seen by a single prism. 



In order to observe the spectrum, the rays of the sun must not 

 be allowed to fall direct upon the prism, for the brilliancy of the 

 light is greater than the eye can bear. The spectroscope should be 

 directed to a wall upon which sunlight is falling, or to the open 

 sky, or a bright cloud. On cloudy days the lower portions of the 

 sky are not sufficiently bright, and as the prism might possibly be 

 displaced if tbe apparatus were held straight upwards, it is best to 

 fix a small mirror in the retort stand, and to give it by repeated 

 trials a suitable position, so that the light of the sky or of a bright 





