173 



SEVENTEENTH CENTURY. 



PT. III. 



But if the wave goes into the glass obliquely (see p. 47), or 

 into a glass with a rounded edge, one end of it will reach the 

 glass first before the other, and will move slowly, while the 

 other end goes on unchecked, and so the wave will swing 

 round and will have its direction altered. In the same way, 

 when it passes out again from the glass, one end will pass 

 out first, and will move more easily in the air than the end 

 that is still in the glass, and so it will swing round again 

 and make another bend. 



This is somewhat difficult to understand at first sight, 

 and it will be best explained by a very ingenious experiment 

 proposed by Mr. E. B. Tylor. 1 Take two small wheels about 



FIG. 35. 



Figures illustrating the passage of the waves of light through different- 

 shaped lenses (Tylor). 



2 inches round, and mount them loosely upon a stout iron 

 axle measuring about half-an-inch round. This will make a 

 runner like two wheels of a cart, and if you let it roll down 

 a smooth board it will represent very fairly the crests or tops 

 of the waves of light in the ether. Let your board be about 

 2 1 feet long, and at one end of it glue on pieces of thick- 

 piled velvet of the shape of lenses (see i, 2, 3, Fig. 35). 

 Let your runner first go straight down the board upon the 

 oblong velvet, it will then run through the velvet without 

 changing its course, as a vertical ray does through a lens. 

 Then start it obliquely, as at a, across the board, so that it 



1 'Nature,' vol. ix. 1874, p. 158. 



