LIGHT. 



223 



LIGHT. 



AMONG the many marvellous results of the labors of the human mind directed 

 to the discovery of the laws of the physical creation, there is perhaps none 

 which strike us with more astonishment than the knowledge which has been 

 obtained relating to the qualities and laws of LIGHT. The principles which 

 govern its reflection from opaque surfaces, and its transmission through trans- 

 parent bodies, we shall examine on another occasion. I propose for the pres- 

 ent to bring before you the facts which have been disclosed regarding its 

 physical nature and its motion through space, as well as the manner in which 

 it affects the organ of vision, so as to produce the perception of external and 

 distinct objects. 



Between the eye and any distant object, there intervenes a space of greater 

 or less extent, and often, as in the instance of the stars, so great as to be 

 scarcely capable of being clearly and adequately expressed by any standard or 

 modolus of magnitude with which we are familiar. Yet objects, at these im- 

 mense distances, are rendered visible to us by some physical effects which 

 they are capable of producing and which in fact they do produce upon our 

 organs of vision. 



We shall see that the interior of the eye-ball is lined with a membrane 

 highly susceptible of mechanical vibration and connected by a continuity of 

 nerves with the brain ; and to this membrane admission is given for light by 

 an opening in front of the eye called the pupil. The light then proceeding 

 from any distant object must be supposed to pass over the space intervening 

 between the object nd the eye, to enter the pupil and to produce upon the 

 membrane within the eye a specific mechanical effect, which being propagated 

 to the brain, is the means of producing in the mind a perception of the distant 

 object. 



How then are we to conceive that an object placed at any distance, for ex- 

 ample, say one hundred millions of miles, from the eye, can transmit over and 

 through that space a mechanical effect which shall be impressed on the eye ? 



