THE LAW OF REFRACTION 



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in ancient times. C is a coin resting on the bottom of an empty- 

 basin, E an eye which cannot see the coin owing to the edge 

 of the basin at B coming between. But, when the basin is 

 filled to the top with water, the ra^'-s from the coin to the eye 

 are refracted and follow the path CAE ; hence the coin becomes 

 visible. 



In order to measure the refraction at different angles, 

 Ptolemy employed a circle divided into 360°, the lower half 

 of which was immersed in water up to the diameter. The 

 centre of the circle was marked by a small coloured body, a 

 similar body was fitted to one of the quadrants out of the water, 

 and a third slid on the lower part which was immersed in the 

 water. This last body was pushed with a rod, until an eye 

 placed in air saw all three in a straight fine. The results are 

 given in the first two columns of the following table : 



In order to investigate the refraction from air to glass, 

 Ptolemy used a semi-cylinder of glass, and arranged it with its 

 plane surface horizontal. The graduated circle was then fixed 

 with its diameter in the plane surface, and observations were 

 made in the same way as before by getting three objects into 

 line. Finally, the semi-cylinder of glass was placed above a 

 water surface, and the refraction from water to glass investigated. 

 The results from air to glass and water to glass are given in the 

 fourth and fifth and seventh and eighth columns of the table. 



The experiment is noteworthy as being one of the few 

 achievements in physics which we owe to the Greeks. The 

 other important ones, namely, the investigation of the laws of 

 vibrating strings by Pythagoras and the discovery of frictional 

 electricity by Thales, occurred some six centuries earlier. As 

 is well known, the Greeks could observe and reason, but were 

 singularly averse to experimental investigation. If we work 

 out the numerical values of the indices of refraction from 



