50 Mr. James Rice on 



much scientific value. For one thing, the eye is quite incapable 

 of distinguishing between two radiations producing the same 

 colour sensation, which a modern spectrometer will resolve with 

 the greatest ease ; for another, as is well known, the particular 

 qualities of radiant energy which are capable of affecting the 

 retina constitute a very narrow range of the totality of ethereal 

 vibrations. 



We can probably approach the question in hand by con- 

 sidering a wave motion in a medium where we can follow the 

 occurrences with great ease. Picture a small needle with its 

 point just touching the surface of a pool of water, and compelled 

 by some mechanism to oscillate up and down ; a disturbance is 

 created in the water, and alternate crests and troughs travel out 

 with a definite speed from the centre of disturbance. There are 

 obviously three quantities of immediate importance ; one is the 

 frequency of oscillation of the needle or disturbing mechanism, 

 i.e., the number of vibrations executed per second ; the second is 

 the wave length of the train of waves sent out — that is, the 

 distance from crest to crest, or trough to trough, or, in general, 

 between two successive rings of water in the same phase of 

 motion ; the third is the speed at which the rings travel out. Of 

 course, when we speak of the rings travelling out, we are refer- 

 ring to an outward movement , not of the water itself, but of a 

 particular form or configuration of the water. As far as the 

 water is concerned, each drop of it is rising and falling like the 

 needle point, and with the same frequency. Very little experi- 

 menting is needed to prove to us that the more frequent the 

 oscillations of the needle the shorter is the Avave-length of the 

 ripples. Indeed, a little thought shows that the two are very 

 simply connected, that in fact the wave-length is the distance 

 which the disturbance advances outwards while the needle 

 executes one complete up and down oscillation, so that the wave- 

 length is determined by a division of the speed of the ripple 

 movement by the frequency of the needle's oscillation. It is 

 clear that for such simple and almost ideal wave motions as these 



