THE FORMS OF WATEK IN 



§ 3. The Waves of Light. 



20. But what is the sun ? We know its size and its 

 weight. We also know that it is a globe of fire far 

 hotter than any fire upon earth. But we now enter 

 upon another enquiry. We have to learn definitely 

 what is the meaning of solar light and solar heat ; in 

 what way they make themselves known to our senses ; 

 by what means they get from the sun to the earth, and 

 how, when there, they produce the clouds of our atmo- 

 sphere, and thus originate our rivers and our glaciers. 



21. If in a dark room you close your eyes and press 

 the eyelid with your finger-nail, a circle of light will be 

 seen opposite to the point pressed, while a sharp blow 

 upon the eye produces the impression of a flash of light. 

 There is a nerve specially devoted to the purposes of 

 vision which comes from the brain to the back of the 

 eye, and there divides into fine filaments, which are 

 woven together to a kind of screen called the retina. 

 The retina can be excited in various ways so as to pro- 

 duce the consciousness of light ; it may, as we have 

 seen, be excited by the rude mechanical action of a blow 

 imparted to the eye. 



22. There is no spontaneous creation of light by 

 the healthy eye. To excite vision the retina must be 

 affecled by something coming from without. What is 

 *.hat something ? In some way or other luminous 



