THE BEASON WHY. HI 



* But if thine eye be evil, thy whole body shall be full of darkness. If therefore 

 the light that is in thee be darkness, how great is that darkness."- MATT. v. 



471. Why are there so many varieties of colour- and tint in 

 the various objects in nature ? 



Because every surface has a peculiar constitution, or atomic 

 condition, by which the light falling upon it is influenced. In 

 tropical climates, where the brightness of the sun is the most 

 intense, there the colours of natural objects are the richest ; the 

 foliage is of the darkest green ; the flowers and fruits present the 

 brightest hues; and the plumage of the buajs* is of the most 

 gaudy description. In the temgerate climates these features are 

 more subdued, still bearing relation to the degree of light. 

 And at a certain depth of the ocean, where light penetrates 

 only in a slight degree, the objects that abound are nearly 

 colourless. 



It has been held by many philosophers (and the theory is so far conclusive 

 that it cannot be dispensed -with) that there is an analogy between the vibra- 

 tory causes of sound, and the vibratory causes of colour. Any one who has seen 

 an yEolian harp, and listened to the wild notes of its music, will be aware that 

 the wires of the harp are swept by accidental currents of air ; that when 

 those currents have been strong, the notes of the harp have been raised to the 

 highest pitch, and as the intensity of the currents has fallen, the musical sounds 

 have deepened and softened, until, with melodious sighing, they have died 

 away. No finger has touched the strings ; no musical genius has presided 

 at the harp to wake its inspiring sounds ; but the vibration imparted to the 

 air, as it swept the wires, has alone produced the chromatic sounds that have 

 charmed the listener. If, then, the varied vibrations of the air are capable of 

 imparting dissimilar sensations of sounds to the ear, is it not only possible, but 

 probable, that the different vibrations of light may impart the various sensa- 

 tions of colours to the eye 1 



CHAPTER XXIII. 



472. What is the refraction of light ? 



"When rays of light fall obliquely upon the surface of any 

 transparent medium, they are slightly diverted from their course. 

 This alteration of the course of the rays is called refraction, 

 and the degree of refraction is influenced by the difference 

 between the densities of the mediums through which light it 

 transmitted. 



