CH. XXVI.] THE SPECTKOSCOPE 433 



a prism, it is refracted or bent at each surface of the prism; the 

 whole ray is, however, not equally bent, but it is split mto its 

 constituent colours, which may be allowed to fall on a screen. The 

 band of colours beginning with the red, passing through orange, 

 yellow, green, blue, and ending with violet, is called a spectrum : this 

 is seen in nature in the rainbow. It may be obtained artificially by 

 the glass prism or prisms of a spectroscope. 



The spectrum of sunlight is interrupted by numerous dark lines 

 crossing it vertically, called Frauenhofer's lines. These are perfectly 

 constant in position and serve as landmarks in the spectrum. The 

 more prominent are A, B, and C, in the red ; D, in the yellow ; E, I, 

 and F, in the green ; G and H, in the violet. These lines are due to 

 certain volatile substances in the solar atmosphere. If the light 

 from burning sodium or its compounds is examined spectroscopically, 

 it will be found to give a bright yellow line, or, rather, two bright 

 yellow lines very close together. Potassium gives two bright red 

 lines and one violet line ; and the other elements, when incandescent, 

 give characteristic lines, but none so simple as sodium. If now the 

 flame of a lamp is examined, it will be found to give a continuous 

 spectrum like that of sunlight in the arrangement of its colours, but 

 unlike it in the absence of dark lines ; but if the light from the lamp 

 is made to pass through sodium vapour before it reaches the spectro- 

 scope, the bright yellow light will be found absent, and in its place a 

 dark line, or, rather, two dark lines very close together, occupying 

 the same position as the two bright lines of the sodium spectrum. 

 The sodium vapour thus absorbs the same rays as those which it itself 

 produces at a higher temperature. Thus the D line, as we term it in 

 the solar spectrum, is due to the presence of sodium vapour in the 

 solar atmosphere. The other dark lines are similarly accounted for 

 by other elements. 



The large form of spectroscope (fig. 367) consists of a tube A, 

 called the collimator, with a slit at the end S, and a convex lens at 

 the end L. The latter makes the rays of light passing through the 

 slit from the source of light, parallel : they fall on the prism P, and 

 then the spectrum so formed is focussed by the telescope T. 



A third tube, not shown in the figure, carries a small transparent 

 scale of wave-lengths, as in accurate observations the position of any 

 point in the spectrum is given in the terms of the corresponding 

 wave-lengths. 



If we now interpose between the source of light and the slit S a 

 piece of coloured glass (H in fig. 367), or a solution of a coloured 

 substance contained in a vessel with parallel sides (the haematoscope 

 of Herrmann), the spectrum is found to be no longer continuous, but 

 is interrupted by a number of dark shadows, or absorption bands 

 corresponding to the light absorbed by the coloured medium. Thus a 



2 E 



