8 TETBEINAKY OPHTHALMOLOGY. 



of the extrinsic muscles the bulbi are rotated so exten- 

 sively and with such harmony that the field is quite 

 extensive. Where rays of light proceed from a lumi- 

 nous body, they always pass in straight lines, forming 

 in their divergence a cone, the apex of which is the 

 luminous body, and the base any plane which may in- 

 tercept them. So long, then, as they travel in a medium 

 of uniform density, so long will they travel in straight 

 lines. Rays passing from a rarer to a denser medium 

 are bent toward the perpendicular at the point of inci- 

 dence. (Fig. 3.) Should they fall upon ?kpol\shed sur- 

 face perpendicularly, they will be refiected in a straight 



line. If obliquely, 

 they will be reflected, 

 and the angle of re- 

 flection is equal to the 

 angle of incidence. 

 (Fig. 4.) If they pass 

 ^'^■*' from a denser to a 



rarer medium, they will be bent from the perpendi- 

 cular (see Fig. 3). 



If a luminous ray passes through a piece of glass, 

 the ray striking obliquely, it will be bent toward the 

 perpendicular, but, on its passing from the glass to the 

 air (denser to rarer), it will be bent away from the per- 

 pendicular. We have seen rays of light passing 

 through plane surfaces. Let us see it through curved 

 surfaces. It is supposed the circumference of a circle 

 is made up of a number of small, straight lines. Take 



