INTRODUCTION TO PHYSIOLOGICAL OPTICS 411 



angles to the pencil of light. Neglect the re- 

 flected rays. 



Observe that the incident rays pass through 

 the bottle and its contents, and are not bent from 

 their course. Light, passing from one medium 

 into another of different density, is not refracted, 

 provided the course of the ray be perpendicular 

 to the surface separating the media. 



2. Turn the bottle .so that the incident ray 

 shall enter it at an angle. 



On passing from the air into the denser 

 medium of the glass and the contained liquid, 

 the incident ray will be bent from its course. 

 On passing from the denser medium into the air 

 again, the ray will once more be bent from its 

 path. Imagine a perpendicular erected at the 

 points of incidence and emergence. The re- 

 fracted ray will be bent toward the perpendicu- 

 lar on passing into the denser medium, and away 

 from the perpendicular on leaving the denser 

 medium. 



Turn the bottle and thus alter the angle be- 

 tween the incident ray and the perpendicular 

 (angle of incidence). 



The angle between the refracted ray and the 

 perpendicular (angle of refraction) increases with 

 the angle of incidence. Exact measurements 

 made by Snellius and Descartes, about 1621, 



