SOME THOUGHTS ON LIGHT. 239 



condition of incandescence. Artificial sources of light are depen- 

 dent for the production of light-rays, upon this incandescent 

 condition. In artificial light we have those bodies which produce 

 flame, and those which have no appearance of flame. Bodies 

 that do not produce light can only be seen by the rays they 

 reflect from some other illuminant. Light may proceed from a 

 self-luminous body or from an illuminated object, but its rays 

 must penetrate the eye before any sensation is produced. 



One of the most curious phenomena connected with light 

 is what is called aberration, in virtue of which all heavenly bodies 

 appear a little out of their true place. The difl"erence in the 

 densities of the atmosphere surrounding our globe and the ether 

 pervading all space, have a tendency to refract the light-rays pro- 

 ceeding to the earth, thus causing us to imagine a body to be 

 w^iere it is not. The motion of light in straight lines is probably 

 the first physical fact the student learns, and it is to the exceptions 

 to this law that every variety of ocular illusion is referable. 



By studying the rectilinear motion of light, we learn that what 

 is called the pencil of light, w^hich emanates from any given point, 

 diminishes in amount of illumination in proportion to the square 

 of the distance from the source of illumination. Of course the 

 above is only true so far as free space is concerned, and when we 

 consider that a portion of light is absorbed in passing through air, 

 we understand at once that the above remarks do not always apply. 



With an elementary knowledge of this subject we very soon 

 find out the fact, that when the action of light is propagated 

 through any medium, and which eventually arrives at another 

 medium of different density, its force is divided and a portion only 

 enters the new medium, whilst the remainder rebounds or reflects 

 backwards. When this rebounding or reflected portion is con- 

 siderable in amount, the surface or point of reflection appears white, 

 but when all the rays of light are absorbed or allowed to pass 

 through the medium the object appears black. 



We learn from a study of this law of reflection, that opacity 

 and transparency are not opposite properties, but rather different 

 degrees of one and the same law, for just as radiant heat of a given 

 intensity penetrates farther through some media than others before 

 becoming entirely absorbed, so with light. Solar rays can penetrate 



