300 



SCIENTIFIC AMUSEMENTS. 



convenient table or stand, with a candle or other source of light 

 in front of it and a mirror, a, lying flat on the table, below the level 



of the surface of 

 the water. A 

 beam of light re- 

 flected from this 

 mirror will pass 

 along the line in- 

 dicated ; it will 

 be refracted on 

 entering the water 



Fig. 139. Total Internal Reflection. te"u y " 



fleeted at the upper surface ; and again refracted on passing out 

 into the air at the other side. 



Fig. 140 represents an analogous case; the glass vessel is filled 

 half full of water, and benzene poured on the top; these two 

 liquids will not mix, whilst the refractive index of benzene is 

 much greater than that of water; accordingly, a ray of light, 

 striking the side of the beaker slantingly downwards, will be 

 refracted, and then totally reflected from the common surface of 

 the water and benzene, and finally refracted outwards again as 

 indicated. 



Expt. 338. The Camera Lucida. The camera lucida is a small 

 instrument intended for making drawings of various objects, so 

 arranged that the eye sees simultaneously the paper and pencil 

 point used for the drawing and the reflected image of the object, 

 formed by one or more internal reflections from a suitably shaped 

 mass of glass. Fig. 141 represents Wollastoris camera lucida. 



Fig. 140. Total Internal 

 Reflection. 



Fig. 141. Wollaston's Camera 

 Lucida. 



where the glass " prism " is so shaped that the angle A is a right 

 angle, and the angle C is 135, so that both the angles B and D are 



