EFFECTS OF THE INDUCTION COIL. 755 



current lasts; but by the law of persistence of visual im- 

 pressions the luminosity appears to be continuous. This 

 may easily be shown by the revolving mirror, fig. 235 

 (page 397). If the tube is placed vertically and its 

 reflected image is observed in the rotating box, a series 

 of separated images of the electric light in the tube will 

 make its appearance. 



Geissler's tubes, containing different gases in a state 

 of great rarefaction, and having their central portions 

 straight and very narrow, are also used for studying 

 the spectra of incandescent gases. 



The tube is placed in a vertical position before the slit of the 

 spectroscope ; or it may be simply viewed through the prism, at a 

 distance of about l m from it, the whole being adjusted precisely as 

 in the experiments on the coloured hydrogen flames described pre- 

 viously (see page 491). The wider portions of the tube at both 

 ends, which are less luminous, give a faint indistinct spectrum ; but 

 the central part, which forms a bright luminous line, produces a 

 sharply denned spectrum consisting of single lines, which are images 

 of the tube. Thus, if the tube is filled with rarefied hydrogen, three 

 lines are seen, a red, a green, and a blue. The spectra of other 

 gases are generally much more complex. 



3 c 2 



