2;8 PHYSICAL SCIENCE 



placed the new method In the hands of investi- 

 gators. The Hnes of the spectrum, whether 

 bright or dark, were thought at first to be fixed 

 and constant in position — that is, the modes 

 of vibration of the atoms from which the light 

 proceeded were imagined to be unaffected by 

 any external circumstances. This supposed 

 simplicity has been shown to be illusory. As 

 we have seen, movement of the source and 

 observer, although it may not alter the atomic 

 vibrations, affects the number of them received 

 in any time, and thus changes the refrangibility 

 of the light they emit as it is received by the 

 observer. But other variations, more funda- 

 mental in their origin, are known. Laboratory 

 experiments have shown that the spectral lines 

 alter their character with changes in the physical 

 conditions of the experiments. It was thought 

 that luminous gases evolved only bright, sharp 

 lines. It is now found that the lines may be 

 broadened and softened by an increase in the 

 pressure or the density of the gas, while, in some 

 cases, a simultaneous shift in position may be 

 produced. An intense magnetic field has been 

 shown by Zeeman to result in separation of 

 single lines into two or more components, in 

 this fulfilling the predictions of the electro- 

 magnetic theory of light, which suggests that 

 some such connection is probable. The spectra 

 of elements have long been known to depend 

 on the temperature, the spectrum of the arc 

 discharge often being different from that obtained 

 by the use of a discontinuous spark, while 

 neither correspond with the spectrum of the 

 incandescent vapour existing in the flame of a 



