306 OPTICAL PROJECTION 



produced by holding a piece of glass more or less smoked in 

 front of the slit. Coloured glasses may of course be used 

 ad libitum. 



A very small piece of glass containing didymium or erbium, 

 or a small bottle filled with solution of didymium sulphate, 

 held in front of the slit, gives remarkably sharp and charac- 

 teristic absorption lines, though it can be shown by simple 

 projection that the medium appears nearly clear. 



The analysing power of even absorption spectra, is well 

 illustrated by comparing the absorption of genuine port or 

 claret, with dilute alcohol artificially coloured. A cell filled 

 with diluted healthy blood, and another containing blood 

 poisoned by carbonic dioxide, is another instructive ex- 

 ample. 



Ordinary vapour-absorption is best shown by heating an 

 ordinary sealed tube of iodine, or by a test-tube filled with 

 nitrous oxide, prepared by pouring nitric acid upon a few 

 bright copper turnings in the tube. There are other vapours 

 which, with a trifle more trouble, give good spectra ; but they 

 are too well known to those specially concerned to need 

 mention here. Most coloured vapours give good phenomena, 

 with more of the line or fluted charactei than is shown by 

 the majority of liquids or solids. 



178. Line Spectra. These are given with greatest facility 

 by the electric arc. For bright-line spectra, the positive 

 carbon need not be larger than usual, but should have a small 

 hollow made in it ; and the current should be of rather high 

 E.M.F., so that the arc maybe long. The metallic vapour will, 

 however, lengthen the arc considerably. The comparatively 

 dark arc should be brought into the focus of the condensers, 

 when the line spectrum of the metals shown will appear 

 between two continuous spectra thrown by the carbon points. 

 Sodium, lithium, silver, copper, zinc, and thallium are the 

 metals usually employed. If only sodium or lithium lines 

 are desired, they are fairly shown by soaking carbon poles, 



