20 LIGHT. 



27. REFRACTION. Re- 



fcion? Re ~ fraction is the change of 

 direction which a ray expe- 

 riences in passing obliquely from a rarer 

 into a denser medium, or the reverse. 



28. The figure represents a block of glass, 

 r^am the ftnd shows tne direction which a ray -of 



light would take on entering and emerging 

 from it. On entering, it makes a bend, and passes on 

 through the glass less obliquely ; that is, more nearly in 

 the direction of a line drawn perpendicularly to the sur- 

 face of the glass, and continued through it. On passing 

 out again it would be bent away from such an imagina- 

 ry perpendicular line, and re-assume its previous course. 



29. ANOTHER STATEMENT OF THE LAW.- 



Give another 



statement of As the perpendicular has only an imagina- 

 Y Y ex i stence ? ^ i g perhaps easier to fix in 

 the mind the changes of direction of rays 

 passing in and out at regular surfaces thus : A ray, on 

 entering a denser medium pursues within it a course fur- 

 ther from the nearest portion of the surface than its origi- 

 nal course would be if continued. And a ray entering a 

 rarer medium takes a course nearer the nearest portion of 

 the surface than its original course would be if continued. 

 These statements are true for all plane or uniformly 

 curved surfaces. 



30. ILLUSTRATION. A 

 Illustrate by a com placed m ft tea _ cnpj ag 



represented in the figure, 

 so as to be barely concealed from the 

 eye, will be rendered visible by filling the cup with water. 



