CH. xxvi.] THE SPECTROSCOPE. 431 



seen constitutes an important test for blood pigment. It will be 

 first necessary to describe briefly the instrument used. 



The Spectroscope. When a ray of white light is passed 

 through 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 

 into 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 spectro- 

 scope. 



The spectrum of sunlight is interrupted by numerous dark lines 

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

 fectly 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, b, 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 

 spectroscope, 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 

 thr 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. 364) 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 trans- 

 parent 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. 



