208 



SENSE OF SIGHT. 



Reflection and Refraction of Light. 



this way, the light has attained the exterior of the organ, the farther 

 transmission is effected through different transparent humours, which 

 consequently form so many media. In its course through media, light 

 may remain unmodified: it may proceed in the same straight line; or 



it may meet with an ob- 



Fig. 77. stacle which arrests it 



altogether, or reflects it ; 

 or, again, it may tra- 

 verse media of different 

 natures and densities, 

 and be made to deviate 

 from its original course, 

 or be refracted. 



When a ray of light 

 falls upon an opaque 

 body, as upon a bright 

 metallic or other mirror, 

 it is reflected from the 

 mirror, in such a man- 

 ner, that the angle made 

 by the incident ray with 

 a perpendicular to the 

 surface of the medium 

 at the point of incidence, 

 is exactly equal to that 

 made by the reflected ray with the same perpendicular. Suppose I J 

 to represent a plate of polished metal, or glass, rendered opaque by a 

 metal spread upon its posterior surface, as in the common looking-glass. 

 The rays, proceeding from an observer at K, will be reflected back to 

 him in the same line K C ; that is, in a line perpendicular to C, the 

 point of incidence. The observer will, therefore, see his own image; 

 but, for reasons to be mentioned hereafter, under optical illusions, he 

 will seem to be as far behind the mirror as he really is before it, or 

 at E. Suppose, on the other hand, that the observer is at A, and that 

 a luminous body is placed at B; in order that the rays, proceeding from 

 it, shall impinge upon the eye at A, it is necessary that the latter be 

 directed to that point of the mirror from which a line, drawn to the eye, 

 atid another to the object, will form equal angles with the perpendicu- 

 lar; in other words, the angle B C K, or angle of incidence, must be 

 equal to the angle of reflection, A C K. In this case, again, the object 

 will not appear to be at B, but in the prolongation of the line A C, at 

 H, as far from the point of incidence C as B is. 



Except in the case of illusions, the study of the reflection of light 

 or catoptrics does not concern vision materially. It is on the princi- 

 ples of dioptrics, that the chief modifications are effected on the pro- 

 gress of the light through the physical part of the organ ; and, with- 

 out a knowledge of these principles, the subject would be totally 

 unintelligible. It is necessary, therefore, to dwell at some length on 

 this topic. 



Whenever a ray of light passes through diaphanous or transparent 



