PART I. IMPONDERABLE AGENTS. 



Sec. I. LIGHT. 

 " LIGHT is THE AGENT OF VISION." 



The history of its mechanical affections belongs to Optics, but some 

 general facts may be advantageously stated here. 



1. ITS MATERIALITY. By some it is supposed to result from 

 the vibration of subtile elastic media ; but every thing goes to counte- 

 nance the idea of its materiality, and this was admitted by Newton.* 



It cannot be weighed, because our balances and organs of sense 

 are not sufficiently delicate. 



2. ITS VELOCITY is two hundred thousand^ miles in a second ; it 

 is seven or eight minutes in coming from the sun, and were its weight 

 the million-millionth, or billionth part of a grain, it would, by its 

 impetus, destroy the firmest bodies. Nine millions of particles of that 

 size would not affect our most delicate balances. J Thorn. 



Momentum, being made up of velocity and quantity of matter, it 

 results, that any degree of momentum may be produced by increas- 

 ing either the quantity of matter, or the velocity ; it therefore follows 

 that the particles of light must be inconceivably small. 



3. Its velocity is progressive, and has been measured, by ob- 

 serving the eclipses of Jupiter's satellites, when the primary is nearest 



Dr. U re has given a different view of this subject. Diet. 3d Ed. p. 563. 



I One hundred and ninety-five thousand. L. U. K. 



\ " The materiality of Light is sufficiently proved. Its motion, though inconceiv- 

 ably rapid, is progressive, and may be measured ; it may be stopped in its progress, 

 or its direction may be changed ; it may be condensed into a smaller, or dispersed 

 over a larger space ; it is inflected when passing near to any body, which proves it 

 to be subject to gravitation ; it produces chemical changes in many bodies, exists in 

 them in a state of combination, and is disengaged by the exertion of new affinities, 

 when it appears in its original form." 



" There is no physical point (says Melville,) in the visible horizon, which 

 does not send rays to every other point ; no star in the heavens which does not 

 send light to every other star. The whole horizon is filled with rays from every 

 point in it, and the whole visible universe with a sphere of rays from every star. In 

 short, for any thing we know, there are rays of light joining every two physical 

 points in the universe, and that in contrary directions, except where opake bodies 

 intervene." A ray of light, coming from any of the fixed stars to the human eye, 

 ' has to pass, in every part of the intermediate space between the point from which 

 it has been projected, and our solar system, through rays of light flowing in all 

 directions, from every fixed star in the universe ; and in reaching this earth, it has 

 passed across the whole ocean of the solar light, and that light which is emitted from 

 the planets, satellites and comets. Yet in this course its progress has not been in- 

 terrupted." Mur. 



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