THE BLOOD 



93 



absent, and in its place a dark line, or rather two dark lines very 

 close together, occupying the same position as the two bright Hnes 

 of the sodium spectrum. The sodium vapour thus absorbs the same 



Fig. 32. — Diagram of spectroscope. 



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 



Fig. 33.— Spectroscope : A, eolliiuator with adjustable slit at one (left) end and collimating lens at 

 the other (right) end ; B, telescope moving on graduated arc divided into degrees ; C, prism or 

 combination of prisms ; D, tube for scale ; E, mirror for Uluminating scale ; F, vessel with 

 parallel glass sides for holding fluid, shown with the flat side towards the reader ; I, long spectro- 

 scope bottle for examining a deep layer of fluid ; H, Argand burner ; G, condenser for concen- 

 trating the light from H on the slit. (From a photograph taken by Dr. MacMonn for 

 McKendrick's ' Physiology.') 



of sodium vapour in the solar atmosphere. The other dark lines are 

 similarly accounted for by other elements. 



The large form of spectroscope (fig. 32) consists of a tube A, called 

 the collimator, with a sht at the end S, and a convex lens at the end L. 

 The latter makes the rays of Ught passing through the sht from the 



