294 SCIENTIFIC AMUSEMENTS. 



almost glancingly along the surface, so that the angle of incidence 

 is not far from a right angle, very little is transmitted and almost 

 the whole reflected. The power of glass to reflect as well as to 

 transmit light is utilised in the optical illusion commonly known 

 as " Pepper's Ghost " (Expt. 340), and in Amici's Camera Lucida 

 (Expt. 338). 



Expt. 333. To Illustrate the Law of Refraction. Place a 

 coin, such as a penny or shilling, at the bottom of a shallow cup 

 or ordinary basin, and then move away until the coin just dis- 

 appears from view, being hidden by the rim of the cup or basin. 

 Now let the vessel be filled with water by some one else, yourself 

 remaining perfectly stationary ; as soon as the surface of the water 

 comes to rest, you will see that the coin is distinctly visible, so 

 that the presence of the water in the cup enables you, as it were, 

 to see round a corner. Fig. 136 represents the track of two of the 



Fig. 136. Refraction of Light. Fig. 137. Law of Refraction. 



rays of light scattered from the surface of the coin which meet 

 the eye and enable the coin to be seen; whilst traversing the 

 water they pass in straight lines, mi; after passing out of the 

 water at ii, they again travel in straight lines, m, but the direc- 

 tion of these lines is not the same as before; at ii, where the 

 rays pass from one medium (water) into another (air), they suffer 

 "refraction" or deviation from the original paths, so that the 

 directions before and after refraction are inclined to one another 

 at an angle, instead of being one and the same line. Accordingly 

 the coin is seen as though it were situated at n. The angle thus 

 made depends on the nature of the media, always obeying a parti- 

 cular geometrical law, which may be thus described. Let the line 

 abCj fig. 137, represent the common surface where the two media 

 join, and let the light from a candle or other source of light, d, 

 strike this surface at the point b, so as to pass after refraction 



