THE SCOPE OF R.\DIOLOGV. 355 



A ray of visible light falling obliquely on tiie surface of a 

 prism of glass or quartz is deviated from its original path, and 

 this deviation is different for different kinds of lig'ht ; hence, 

 when composite light falls on the i^rism, the constituent parts 

 will be deviated in definite directions, and form a series of 

 colours on the receiving screen. This series of colours or of 

 bright lines is called the spectrum of the light. 



Iif a bar of metal is gradually heated and its s])ectrum 

 observed, the following phenomenon is seen : Initially no visible 

 light appears, but heat is radiated — that is, invisible light is given 

 ■off'. Subsequently the metal begins to glow, and a dark red liiiht 

 is seen in the spectrum. As the heating is continued, the follow- 

 ing additional colours appear consecutively : Orange, yellow, 

 green, blue, and finall\- violet, where the visil)le s])ectrum ends. 



The visible region of the spectrum forms only a small frac- 

 tion of the complete s|)ectrum of radiations. This region extends 

 from the wave-length 0.0008 millimetre — ^the red end — to 

 wave-length 0.0003 millimetre — the violet end. Beyond 

 the red end, we have the long wave-length heat rays (0.06 

 millimetre), and the Hertzian or electro-magnetic rays (10,000 

 metres). Beyond the violet end, we have the short actinic 

 (Schumann) rays (o.oooi millimetre), and the A^-rays 

 (o.ooooooooT millimetre). 



From what has been said about the radiations given oft' 

 l)y an incandescent solid, we notice that the short wave-lengths 

 appear at a higher temperature than the long wave-lengths ; 

 hence, by observing the spectra of stars, we may form some idea 

 of the temperature of the stars, and by comijaring tlie spectra 

 we may construct a celestial thermometer. 



The spectrum of an incandescent solid is one continuous 

 band of colour, whereas an incandescent gas generally gives a 

 spectrum of definite bright lines only, and these lines give us a 

 clue to the nature of tlip .o^as. 



These bright lines are replaced by dark lines in the spec- 

 trum if the light ivom an incandescent solid jjasses through a 

 layer of the same gas at a lower temperatin^e ; thus the dark 

 lines in the snectra of stars indicate the presence of definite 

 gases between the observer and the star. Hence the tempera- 

 ture and constitution of a star are revealed in its spectrum. 

 When the source of light moves relatively to the observer, the 

 waves of light are compressed or expanded, hence there is an 

 apparent change in the wave-lengths; conse(|uently the "shift 

 of the lines" indicates the direction of motion of the source; 

 thus the temperature, constitution, and motion of celestial bodies 

 are shown in tlie si^'X'tra. 



Just as a gas has a definite absorption spectrum under 

 definite conditions, so, too, li(|uids selectively absorb definite 

 radiations when light is transmitted through them. The energy 

 of the radiations which are abs(ubed may l)e used to cause new 

 radiations. An instance of this is fluorescence. 



This selective absorption is met with in the ]:)igments of 



