xi LIGHT 161 



spreads in every direction until the edge of the pond is reached. 

 Had a cork been placed upon the water, it would have been 

 noticed that when the disturbance reached it, all that happens 

 to it is an up and down motion. It does not move forward with 

 the wave. The downward motion of the water producing the 

 depression when the stone comes into contact with the pond is 

 followed by the production of a crest by the swinging back of 

 the water particles, and this crest moves across the pond 

 immediately after the depression and with the same velocity. 

 This is succeeded by the formation of another depression, which 

 is followed by a second crest, and so on until the effect of the 

 impact of the stone has died away. The distance from one crest 

 to the next or from one depression to the next is called a wave- 

 length. 



It must be very carefully noticed that the water particles 

 themselves move up and doivn, 1 or vertically, as shown by the 

 cork, whereas the wave itself moves along the surface of the 

 water, or horizontally. These two directions are at right angles 

 to one another, and such a wave is called a transverse wave. 



If the motion of the particles is in the same direction as 

 thab of the propagation of the wave motion the wave is called 

 longitudinal. 



Ether- waves are of the first kind, i.e., transverse waves ; 

 sound waves in the air are of the latter kind. 



Ether- Waves are of Three Kinds. The ether- waves which 

 come to our planet from the sun, and comprise what is inclu- 

 sively called sunlight, produce different effects. If they fall 

 upon our bodies they may be absorbed, and the energy of the 

 wave-motion become converted into heat, and for this reason 

 the waves which have been absorbed are spoken of as Radiant 

 Heat ; if they fall upon the retina of an eye, they may produce 

 a sensation of light, and we then call the waves Light ; falling 

 upon a photographic plate or upon a green leaf, they may 

 produce certain chemical effects, and are then referred to as 

 Actinic rays. The word " rays " means simply kinds of radia- 

 tion, and hence we may speak of heat rays, light rays, and 

 actinic rays. It cannot be too strongly insisted that in their 

 passage through the ether these ether- waves do not give rise to 

 any of these results ; they are simply waves transferring energy 

 by wave-motion. 



1 Really each particle of water moves round in a circle, but the statement in 

 the text brings out the nature of transverse waves. 



