LIGHT. 



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 case of the 

 stars, so great as to be incapable of being clearly and adequately 

 expressed by any standard or modulus of magnitude with which 

 we are familiar. Yet objects, at these immense distances, are 

 rendered visible to us by some physical effects which they 

 produce upon our organs of vision. 



It has been ascertained that the interior of the e} r e-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 to 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 and 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 example, say one hundred millions of miles, from 

 the eye, can transmit over and through that space a mechanical 

 effect on the eye ? We answer that there are two, and only two, 

 ways in which it is possible to conceive such an action to take 

 place. These two are the following : 



First. The distant object thus visible to us, may emit particles 

 of matter from its surface, which particles of matter may pass over 

 the intervening space, may enter the pupil of the eye, may strike 

 upon the nervous membrane, and so affect it as to produce vision. 



Secondly. There may be in the space between the distant 

 visible object and the eye, a medium possessing elasticity, so as to 

 be capable of receiving and transmitting pulsations or undu- 

 lations like those imparted to the air by a sounding body. If 

 this be admitted, the distant visible object may, without emitting 

 any particles of matter from its surface, affect such a medium 

 surrounding it with pulsations or undulations, in the same 

 manner as a bell affects the air around it. These pulsations or 

 undulations may pass along the space intervening between the 

 visible object and the eye, in the same manner as the pulsations 

 or undulations produced by a bell pass along the air between the 

 bell and the ear. In this manner, the pulsations transmitted 

 from the visible object, and propagated by the medium we have 

 referred to, may reach the eye and affect the membrane which 

 lines it, in the same manner exactly as the pulsations in the air 

 affect the tympanum of the ear. 

 194 



