The Senses of Animals. 273 



the highest vibrations. Some people are, moreover, 

 relatively or absolutely insensible to certain colours, 

 generally either red or green. Such persons are said to 

 be colour-blind. When the rainbow colours are combined 

 in due proportion, or when pairs or sets of them are com- 

 bined in certain ways, white light is produced. 



We saw that in the case of sound-waves, when the 

 number of vibrations in a second is doubled, the sound is 

 raised in pitch by an octave. Using this term in an 

 analogous way for colour-tones, we may say the range in 

 average vision is about one octave that is, from about 

 400 billion to about 800 billion vibrations in a second. 

 But, though these are the limits in human vision, we know 

 of the existence of many octaves of radiant energy 

 physically in continuity with the light-vibrations. Photo- 

 graphy has made us acquainted with ultra-violet vibrations 

 up to about 1600 billions per second an octave above the 

 violet. And Professor Langley's observations with the 

 bolometer indicate the existence of waves with as low a 

 vibration-period as one billion per second, and even here, 

 in all probability, the limit has not been reached. To the 

 vibrations more rapid than those that are concerned in 

 the sensation of violet, the human organism is apparently 

 in no manner sensitive. But to infra-red vibrations down 

 to about thirty billions per second the nerves of the skin 

 respond through the temperature-sense. We shall have to 

 return to these limits of sensation at the close of this 

 chapter. 



The human eye is a nearly spherical organ, capable of 

 tolerably free movements of rotation in its socket. What 

 we may call the outer case, which is white and opaque 

 elsewhere, is quite transparent in front. Through this 

 transparent window may be seen the coloured iris, in the 

 centre of which is a circular aperture, the pupil. The size 

 of the pupil changes with the amount of light it dilates 

 or contracts, according as the light is less or more intense. 

 Just behind it, and still in the front part of the eye, is the 

 transparent lens, the convexity of the anterior surface of 



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