Discontinuity in the Pheyioinena of Radiation 57 



telescope of the ordinary spectrometer, and the position of the 

 dark lines and bands on the developed plate give the necessary 

 data for measuring the direction of the diiTracted streams of. 

 radiation which produced this effect on the negative. The 

 amount of chemical action at each line on the negative, estimated 

 by the depth of tone, indicates the intensity of the stream of 

 radiation. With the specially sensitized plates available to-day, 

 the range of radiation over which the photographic plate can be 

 used is very wide, but it is for those (|ualities of radiation which 

 are shorter in wave-length than the extreme violet of the visible 

 spectrum, the so-called actinic, photographic or ultra-violet rays, 

 that it has been mainly employed. 



My remarks so far have been based more or less on the 

 assumption that ideal radiations of precise and definite wave- 

 length exist and are detactable. I must qualify that statement 

 somewhat. As already stated, there are certain luminous bodies, 

 such as the incandescent alkali earths, or electrically excited 

 gases, which emit radiations approximating to the ideal form ; 

 but even in such cases, what we actually detect is a stream of 

 radiation with wave-length between certain limits, restricted, no 

 doubt, but still not absolutely identical. The reason for this is 

 not merely the finite dimensions of our receiving and detecting 

 apparatus ; it lies in the very nature of matter itself. But apart 

 from such radiations as are approximately monochromatic or 

 homogeneous, we have in the sun, the flame of a candle, the 

 incandescent metal filament or mantle, the electric arc, and so on 

 a whole series of bodies which are pouring forth radiations which 

 are the very opposite of homogeneous. Even the tumbled surface 

 of a choppy sea can only give an imperfect analogy to the com- 

 plexity of ethereal vibration which must be set up by the presence 

 of an ordinary incandescent solid. Yet the grating and radio- 

 meter can evolve order out of that seeming chaos and discover 

 uniformity of behaviour. The grating will analyse a stream of 

 such heterogeneous radiation into component streams, each stream 

 having its own narrow limits of wave-length or frequency, and 



