8 VETERINARY 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 ?^polislied sur- 

 face perpendicularly, they will be reflected 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 

 l^erpendicular, but, on its passing from the glass to the 

 air (denser to rarer), it will be bent cmay 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 



