CHAPTER IX. 



LIGHT AND ITS SOURCES WHAT IS LIGHT? VELOCITY OF LIGHT 

 REFLECTION AND REFRACTION RELATIVE VALUE OF LIGHTS. 



THE subject of Light and the science of Optics are so interesting to all 

 cf us that some short history of light is necessary before we can enter 

 upon the scientific portion of the subject. The nature of the agent (as we 

 may term light) upon which our sight depends has employed man's mind 

 from a very early period. The ancients were of opinion that the light 

 proceeded from the eye to the object looked at. But they discovered some 

 of the properties of light. Ptolemy of Alexandria, who was born A.D. 70, 

 made some attempts to discover the law of Refraction ; and we are informed 

 that Archimedes set the Roman fleet on fire with burning-glasses at Syracuse. 

 The Arabian treatise of Alhagen, in 1 1 oo A.D., contains a description of the 

 eye and its several parts ; and the writer notices refraction and the effects of 

 magnifying glasses (or spectacles). Galen, the physician, practically dis- 

 covered the principle of the stereoscope, for he laid down the law that our 

 view of a solid body is made up of two pictures seen by each eye separately. 



Still the science of optics made little progress till the law determining 

 the path of a ray of light was made known, and the laws of refraction 

 discovered. Refraction means that a ray is deflected from its straight course 

 by its passage from one transparent medium to another of different density. 

 The old philosophers found out the theory of sound, and they applied them- 

 selves to light. Newton said light consisted of minute particles emanating 

 from luminous bodies. Huyghens and Euler opposed Newton's theory of 

 the emission of light ; and it was not till the celebrated Thomas Young, 

 Professor at the Royal Institution, grappled with the question that the 

 undulating or wave theory of light was found out. He based his investiga- 

 tions upon the theory of sound waves; and we know that heat, light, and 

 sound are most wonderfully allied in their manner of motion by vibration. 

 But he was ridiculed, and his work temporarily suppressed by Mr. Brougham. 



Light, then, is a vibratory motion (like sound and heat), a motion of 

 the atoms of our ether. But how is the motion transmitted ? Sound has 

 its medium, air ; and in a vacuum sounds will be very indistinctly heard, if 

 heard at all. But what is the medium of communication of light ? It is 

 decided that light is transmitted through a medium called ether, a very 

 clastic substance surrounding us. The vibrations, Professor Tyndali and 



