in Energy, the Power of Nature 37 



way, giving rise to echoes, and so are the little ripples of a 

 water surface on meeting a straight line of cliffs. When 

 the ripples of the sea pass among a number of half-covered 

 stones their onward path is changed in direction, each little 

 undulation being bent from its course by the obstacle it 

 meets. Similarly, when a ray of radiant energy passes from 

 one medium into a denser, from the ether into air, or from 

 air to glass, for example, the undulations are diverted by 

 the particles of matter, and the path of the ray is bent or 

 refracted. Radiant energy is made up of many different 

 vibrations ; some are comparatively long and are slow in 

 their vibration, others are very short and much more 

 rapid. The short quickly -vibrating waves are most bent 

 from their straight path by passing into a different medium, 

 and are therefore said to be most refrangible. It is evident 

 that if a beam of radiant energy, in which each ray corre- 

 sponds to a definite wave-length, travelling straight on, 

 enters a denser medium, the separate rays will be spread 

 out like the ribs of a fan, those of the shortest waves being 

 most turned from the straight line, those of the longest 

 waves least. 



62. The Spectrum. When the undulations which come 

 from an intensely vibrating solid enter a triangular glass 

 prism (Fig. 7, P) through a narrow slit, they are spread 

 out by refraction and arranged side by side in perfect order 

 from those of shortest 

 wave-length, -z/, to those 

 of longest, r forming a 

 spectrum. The waves 

 shorter than -g-yj^ of 

 an inch have a peculiar 

 power of affecting certain 

 substances and produc- 

 ing chemical changes, FlG - 7--Pf.T atic ref l action v RR', straight 



5. ' path of light ray ; Rvr, refracted path. 



but they have no effect 



on the senses. The waves between -QJ-^-^ and -jj-g-g-g-g- of 

 an inch in length (vr) affect the sense of sight through the 

 eye, producing the sensations of light and colour, hence 

 they are termed light-waves. Waves of longer wave-length 



