ON LIGHT. 235 



(18.) Before we can give any intelligible account of 

 these theories, however, it is necessary to enter a little 

 more particularly into the modes by which a ray of light 

 may be deflected from its rectilinear path, and the laws 

 of such deflection. By this expression we understand 

 nothing more than that the line of communication between 

 the illuminating and illuminated object is, in some way 

 or other rendered circuitous. It is so natural to speak of 

 light as a thing, and of its line of communication as the 

 path along which that thing, be it what it may, travels, 

 that we are apt to forget that (except on one hypothesis 

 as to its nature, viz., that it is, actually, a material sub- 

 stance, bodily transported from place to place] this form of 

 expression is purely metaphorical, and that by a ray 

 nothing more is meant than the mathematical line, be it 

 straight or bent, between two points, standing to each 

 other in the relations of illuminatwjg- and illuminate/ 

 along which the communication is kept up the test 

 being, that an opake body being placed anywhere in 

 that line, the illumination ceases. Such a circuitous 

 line of communication may be established, independent 

 of and in addition to the direct rectilinear one, by plac- 

 ing anywhere in space any material object whatever, 

 provided there be no opake body interposed between 

 it and either of the two points j and this in two different 

 modes. In the one the whole path of the ray, both be- 

 fore and after its deflection, is outside of the deflecting 

 body. In this case the light is said to be "reflected:" if 

 at a smooth and polished surface, regularly, if at a rough 

 one, irregularly ; in which case the light is said to be 



