ON THE THEORV OF OPTICS. 409 



eye, and of affecting it with a sense of vision. A body, from which this 

 influence appears to originate, is called a luminous body. We <lo not 

 include i n this definition of the term light the invisible influences which 

 occasion heat only, or blacken the salts of silver, although they both appear 

 to differ from light in no other respects than as one kind of light differs 

 from another; and they might probably have served the purpose of light, if 

 our organs had been differently constituted. 



A ray of light is considered as an infinitely narrow portion of a stream of 

 light, and a pencil as a small detached stream, composed of a collection of such 

 rays accompanying each other. As a mathematical line is sometimes conceiv- 

 ed to be described by the motion of a mathematical point, so a ray of light may 

 be imagined to be described by the motion of a point of light. We cannot exhi- 

 bit to the senses a single mathematical line, except as the bountlary of two 

 surfaces; in the same manner, Ave cannot exhibit a single ray of light, except 

 as the confine between light and darkness, or as the lateral limit of a pencil 

 of light. 



When light passes through a space free from all material substances, it 

 moves, with great velocity, in a direction perfectly rectilinear; when also 

 it passes through a material substance perfectly uniform in its structure, it 

 probably always moves in a similar manner. But in many cases its motions 

 are much interrupted. Those substances through which light passes the 

 most freely, and in straight lines, are called homogeneous transparent 

 mediums. Perhaps no medium is, strictly speaking, absolutely transparent; 

 for even in the air, a considerable portion of light is intercepted. Ithas 

 been estimated that of the horizontal sunbeams, passing through about 200 

 miles of air, one two thousandth part only reaches us; and that no sensible 

 light can penetrate more than 700 feet deep into the sea ; a length of seven 

 feet of water having been found to intercept one half of the light which 

 enters it. . 



It is possible that mediums, not in other respects identical, may be homo- 

 geneous with respect to the transmission of light ; for example, a glass may 

 be filled,with a fluid of such a density, that the light may pass uninterruptedly 

 through their common surface; but it generally happens, that whenever the 



VOL. I. ^ 3 G 



