82 THE ANATOMY OF SCIENCE 



the sum of the other two but will be given by 

 the equation 



Vi + V, 



V3 



1 + V1V2 



Thus if each of the two velocities is nearly the 

 velocity of light, or nearly unity, the sum will 

 remain less than the velocity of light. This 

 equation merely restates the law of the addi- 

 tion of slopes that we have found for this non- 

 Euclidean geometry. 



Fizeau, in the middle of the last century, per- 

 formed a very interesting experiment in this 

 connection. Light passing through a medium 

 like glass or water travels with less than the 

 velocity of light (the velocity in free space). 

 Now supposing that we have light passing 

 through a moving stream of water, will the 

 velocity be the same as if the water were at rest, 

 or will it be that velocity plus the velocity of 

 the water? The experiment was tried and gave 

 a result l3^ing between the two, and this was 

 believed to show a partial dragging of the ether 

 with the stream of water. Many pages of mathe- 

 matical physics were devoted to the discussion 

 of why the ether happened to move in just this 



