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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,