SIMPLE EXPERIMENTS IN OPTICS. 



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light: 1. We have a clear glass bottle, with a sheet of white paper, in 

 which a perfectly round hole has been cut, pasted on its side ; 2. Hori- 

 zontal and perpendicular lines are drawn with ink upon the glass at 

 right angles across each other, and within this circle, dividing it into 

 four equal parts ; 3. Water is poured into the bottle until its level is 

 that of the horizontal line ; 4. A postal-card containing a slit is placed 

 as at D in the figure ; 5. The mirror, _S, reflects the beam into the 

 bottle so that it may touch the water where the two lines cross. The 

 light is seen to bend as soon as it enters the water. The index of re- 

 fraction for all liquids may be determined by measuring the distance 

 from where the beam enters to the perpendicular, then the distance 

 from the per^^endicular to where it vanishes, and dividing these into 

 each other. The constancy of the quotient for each particular liquid 

 can also be shown by having the beam strike the water at various 

 angles, making these measurements and dividing. No matter how 

 the distances vary from the perpendicular, when divided into each 

 other they give the same result for the same liquid. 



Fig. 6. Total Reflection. 



With a mirror on the table and our bottle arranged as in Fig. 6, 

 total reflection will occur, that is, all the light of the beam will be 

 thrown downward from the surface of the water. 



" Sometimes, when walking along a road on a warm day, you may observe 

 a curious quivering in the air just where the road seems to meet the sky, as it 

 goes over a hill. The objects near this point appear to be distorted and to 

 tremble, or they assume fantastic shapes. Here we have an instance of refrac- 

 tion caused by the heated air just above the surface of the road. The light 

 passing through these layers of unequally-heated air is refracted unequally, and 

 the objects that reflect the light appear distorted. In some instances the refrac- 

 tion may pass the critical angle, and we may see the objects apparently doubled 

 by reflection. Warm, calm days by the sea show the same thing, when distant 



