EXPERIMENTS ON LIGHT-SPECTRA. 489 



not transmit light of only one colour, but always a 

 mixture of various colours, each of which has a different 

 refrangibility ; thus blue glass transmits mostly blue 

 rays, but also some that are green and violet, and 

 even a, sensible quantity of red rays. If, therefore, 

 blue glass is interposed between the slit and the prism, 

 no distinct blue, but an indefinite band, presents 

 itself when the slit is viewed through the prism, 

 the colours extending from green to violet with a 

 reddish tint somewhat beyond the edge. It is, however, 

 possible to show in another way that the coloured 

 spectrum which a bright object (a slit, a flame, &c.) 

 presents when viewed through a prism is really pro- 

 duced by seeing the body in various colours placed side 

 by side. For this purpose a hydrogen flame is used, 

 to which, by introducing various substances into it, 

 different colorations may be given. 



For these experiments the gas-generating apparatus in fig. 156 is 

 used. An india-rubber tube is attached to the stopcock h, and the 

 other end is drawn over the mouthpiece of a blowpipe, so that the 

 gas may issue through the fine aperture. The blowpipe is clamped 

 horizontally in the retort-stand by its longer end, the fine aperture 

 being directed upwards, so that the flame is perpendicular. Before 

 attaching the blowpipe, the gas must be carefully tested for its 

 purity, as in the case of the experiments with the chemical har- 

 monicon. For the experiments to be described, hydrogen should 

 not burn at the end of a glass tube, because the glass by itself, when 

 hot, gives a yellow colour to the flame. Care must also be taken 

 that the aperture of the blowpipe is perfectly clean, as even slight 

 impurities may colour the flame. The latter should be regulated 

 by the stopcock so as to have a height of about 1'5 to 3 cm . When 

 several experiments are to be made in succession, the generating 

 apparatus should be previously filled anew. 



A platinum wire must be used for introducing the substances 

 into the flame ; other metals do not resist the heat of the hydrogen 

 flame sufficiently well. A platinum wire, from 3 to 6 cm long and 



