86 REFLECTED AND TRANSMITTED LIGHT. 



CHAPTER X. 



REFLECTED AND TRANSMITTED LIGHT. 



A GREAT many curious and beautiful illusions are pro- 

 duced by the reflection of light. One of the most remark- 

 able of these is the exhibition of pretended ghosts and hob- 

 goblins at places of public entertainment. John went with 

 Lawrence to witness one of these exhibitions at a place of 

 instruction and amusement in London called the Polytech- 

 nic Institution, where the whole process was explained. I 

 shall presently give an account of this visit, but in the 

 mean time, in order that the reader may clearly under- 

 stand the nature of the phenomenon, it is necessary that he 

 should pay attention for a moment to a certain mathemat- 

 ical principle. 



If you throw a ball from your hand to the floor direcf.li/ 

 downward that is, at right angles to the floor, its tenden- 

 cy is to rebound directly upward that is, to come up as it 

 went down, namely, at right angles. 



On the other hand, if you throw the ball somewhat for- 

 ward, so that it shall strike the floor at some distance be- 

 fore you, it will, in rebounding, go still farther on. In this 

 case the ball strikes the floor at an oblique angle, and, on 

 rebounding, or being reflected, as we might say, it rises at 

 the same angle on the other side. 



It follows from this that if a boy and a girl standing at 

 a little distance apart are playing with a ball, and the boy 

 wishes to throw the ball so that the girl may easily catch 

 it on the rebound, he must throw it so that it shall strike 

 the floor as nearly as possible midway between them, so 

 that it may have a horizontal distance to rise in equal to 



