CHAPTER XXII 

 LIGHT AND VISION 



125. Radiation from Luminous Bodies. When a body is 

 heated very hot, it gives off, in addition to the heat rays, cer- 

 tain other rays which have the power of affecting the retina 

 of the eye, thus producing a sensation which we know as light. 

 All bodies which give out light rays are said to be luminous. 

 The sun is a luminous body, and so are the stars, but the moon 

 is not because it shines by reflected sunlight. From the fact 

 that light passes easily across space in which there is no matter 

 of any kind, we perceive that the light waves are not waves in 

 air. Scientists regard them as waves in a mysterious sub- 

 stance called ether, about which practically nothing is known, 

 but which is supposed to exist between the molecules of matter 

 as well as between the earth and the other heavenly bodies. 

 Though it is almost impossible to study the ether, the waves 

 that occur in it, that is, the heat and light waves, are very well 

 known. The number and direction of their vibrations have 

 been ascertained and their speed has been accurately measured 

 and found to be about 186,000 miles a second. Such a veloc- 

 ity is inconceivable. For all distances on the earth, it is prac- 

 tically instantaneous. In passing from the sun to the earth, 

 however, the distance is so great that about eight minutes are 

 required. We know that light travels in a straight line, be- 

 cause all opaque bodies cast shadows which have the same 

 outlines as the bodies themselves. Moreover, when light 

 enters a dark room through some small opening, we find it 

 always follows a straight path, as is shown by the fact that 



149 



