22 LIGHT. 



of glass which it holds will stand alone, the corners 

 where they meet are also closed with sealing wax. 

 The prism is then filled with water, taking care not to 

 moisten the upper edges, and a glass top is afterward 

 attached by wax. 



34. ACTION OF THE 

 Explain the LENS The property 



action of the A 



convex lens. which a COI1V6X lens pOS- 



sesses of converging rays 

 of light and heat, and bringing them 

 together in a point, is also a consequence of refraction. 

 All of the rays which fall upon its surface are bent, as 

 shown in the case of the prism ; but, owing to its shape, 

 they are bent in diiferent degrees and directions, so that 

 they all meet in a point. This point is intensely 

 bright if brought on a dark object, and is called the 

 focus. 



35. There is another law of refraction 



Explain ano- . '* 



ther law of which has not been stated, which is essen- 

 tial to a full understanding of the convex 

 lens. According to this law, the more obliquely a ray 

 falls upon any surface the more it is refracted or bent out 

 of its course. And it is a consequence of the shape of the 

 lens, and its greater steepness toward the edge, that of 

 all the parallel rays which fall upon its surface, those 

 which fall furthest from the center fall most obliquely, 

 and enter the air again more obliquely. In proportion, 

 therefore, as they need to be bent to be brought to the 

 focus, they are thus bent by the action of the lens. 



36. ANALYSIS OF LIGHT. It has, up to 

 cometed i- ^is point, been assumed that light is sim- 



