HERMANN VON HELMHOLTZ 



not only what is commonly regarded as space, but 

 also all translucent substances. By the molecular 

 movements of luminous bodies, this ether is set vibrat- 

 ing in a series of waves. The ether vibrations that 

 constitute these waves may be conceived to be at 

 right angles to the direction of the ray of light, just 

 as the surface of calm water, which has been agitated 

 by a stone, rises and falls as the waves spread outwards. 

 Thus a cork floating on the water, traversed by a 

 wave, oscillates up and down nearly at right angles to 

 the direction of the wave. These wave-like move- 

 ments of the ether impinging on the retina set up in it 

 changes which result, after their effect has been trans- 

 mitted to the brain, in the sensation of light, but the 

 sensation in no way resembles its physical cause, 

 although it varies with variation of the stimulus. 1 

 The intensity of the sensation varies with the ampli- 

 tude of the wave. Large waves give rise to a sensa- 

 tion of bright light, small waves to a sensation of dim 

 light. Again, the sensation of colour depends on the 

 rapidity with which the waves follow one another, or, 

 in other words, on the length of the wave. This 

 rapidity, though inconceivably great, may still be 

 accurately determined. Ordinary sunlight, as Newton 

 showed, is composed of a series of colours (using the 

 word in an objective sense) blended together, but yet 

 separable from one another, because each colour is due 



1 Physiology of the Senses. By M'Kendrick and Snodgrass. London. 

 1893, p. 115. 



114 



