238 SOME THOUGHTS OX' LIGHT. 



light under proper condition produces darkness. Now, on the 

 supposition of the first or emission theory, the fact that light added 

 to light produces darkness, is wholly inexplicable, whilst if we 

 accept the latter or undulatory theory, the matter becomes easy to 

 explain. Therefore, setting aside the view that light is itself 

 material, it becomes necessary for us to consider the nature of the 

 medium by which such a phenomenon is transmitted to us. 



We are all aware that our atmosphere forms but a compara- 

 tively thin envelope around the surface of this globe, and when we 

 consider that the light of all heavenly bodies is transmitted to us 

 by undulation, we are forced to admit that it must be through 

 some other medium than the atmosphere. Hence, it is inferred 

 that the whole universe is filled with an elastic rarified substance 

 which opposes no appreciative resistance, and this material has 

 been termed ether. 



According to Prof. Huyghens, light consists of very minute 

 vibrating movements of this elastic ether, such movements being 

 propagated with great rapidity — but not instantaneously — in 

 straight lines, which proceed like the radii of a sphere from a 

 central point common to all. Originally, the idea was accepted 

 that light was a direct propagation along a single straight line, 

 thus allowing of a possible isolated ray, but the existence of an 

 isolated ray is now held to be inconceivable. 



It may be interesting to consider some of the elementary 

 effects of light, such as its source, its direction of transit, and the 

 aberrations which it undergoes. We will then proceed to 

 investigate the first branch of optical science, viz.. Perspective, 

 and afterwards make some reference to Caloptrics. Light, as we 

 have previously stated, travels in straight lines, radiating in all 

 possible directions from every luminous point. 



In the science of light we find that there arises a twofold con- 

 sideration : first, we must investigate the laws relating to light 

 itself, and then secondly, we require to examine the phenomenon 

 of vision. As to the source of light, we must point out that it 

 seems to be very closely associated with heat, for every form of 

 matter, when sufficiently heated, possesses the power to emit rays 

 of light. 



It is probable, therefore, that self-luminous worlds are in a 



