LIGHT: REFRACTION 



279 



space above may be filled with a whiff or two of smoke ; or, 

 (b) a piece of white card or painted tin leaned a little slanting 

 against the farthest side, will answer the same purpose and 

 avoid scattering any light. Arrange the lantern with a 

 horizontal slit on the nozzle N, and parallel beam alone if the 

 tank is brought close for a single experiment ; but it is often 

 more convenient to have it farther away, when the focussed 

 beam should be employed. By the plane-mirror adjusted on 

 its stand behind the tank, the beam may be first thrown down 

 perpendicularly ; then through a slit at F near the end of the 

 tank, when it will 

 be seen that the ray 

 is bent down or re- 

 fracted ; and finally 

 by tilting the lantern 

 a little behind, the 

 rays may be sent into 

 the tank through 

 the glass end at B, 

 just above the water, 

 when it will be seen 

 to be far more bent 

 down, to c. In both 

 cases the reflected portion of the rays will also be seen. 



158. Total Reflection. By having a slit in the bottom of 

 the tank covered with a piece of glass, the rays may be easily 

 sent up from water into air, when it will be seen that the rays 

 are then refracted away from the perpendicular, the ray in 

 either case being exactly reversible. The arrangement will 

 be very similar to that shown in fig. 147, which depicts the 

 apparatus adjusted to demonstrate total reflection. By tilting 

 the bottom reflector the curiously instantaneous character of 

 the transition from transmission to total reflection within the 

 water, is readily shown. 



A beautiful experiment in total reflection is known as the 



FIG. 146. Refraction 



