574 



Professor Oliver Lodge 



[April 1, 



Fig. 7. 



i 



''U 



//2 



were formed on screen by three mirrors, one of them semi- 

 transparent, as in Fig. 7.] 



Now a most interesting and important, and I think now well- 

 known experiment of Fizeau proves quite simply and definitely 



that if light be sent along a stream 



of water, travelling inside the water 



as a transparent medimn, it will go 



quicker with the current than against 



it. You may say that is only natural ; 



a wind helps sound along one way 



and retards it the opposite way. 



Yes, but then sound travels in air, 



and wind is a bodily transfer of air, 



hence, of course, it gives the sound a 



y I I ride ; whereas light does not really 



y^C_____^ / travel in water, but always in ether. 



/X^S " 7^--—-^ It is by no means obvious whether 



a stream of water can help or hinder 

 it. Experiment decides, however, and 

 answers in the affirmative. It helps 

 it along with just about half the speed 

 of the water ; not with the whole 

 speed, which is curious and impor- 

 tant, and really means that the moving 

 water has no effect whatever on the 

 ether of space, though it would take 

 too long to make clear how this 

 comes about. Suffice for present pur- 

 poses the fact that the velocity of 

 light inside moving water, and there- 

 fore presumably inside all transparent matter, is altered by motion 

 of that matter. 



Does not this fact afford an easy way of detecting a motion of the 

 earth through the ether ? Here on the table is water travelling along 

 nineteen miles a second. Send a beam of light through it one way and it 

 will be hurried ; its velocity, instead of being 140,000 miles a second, 

 will be 140,009 miles. Send a beam of light the other way, and its 

 velocity will by 139,991 ; just as much less. Bring these two beams 

 together ; surely some of their wave-lengths will interfere. M. Hoek, 

 Astronomer at Utrecht, tried the experiment in this very form ; here 

 is a diagram of his apparatus (Fig. 8). Babinet had tried another 

 form of the experiment previously. Hoek expected to see interference 

 bands, from the two half-beams which had traversed the water, one in 

 the direction of the earth's motion and the other against it. But no 

 interference bands were seen. The experiment gave a negative 

 result. 



An experiment, however, in which nothing is seen is never a very 

 satisfactory form of a negative experiment ; it is, as Mascart calls it, 



Plan of Interference Kaleido- 

 scope. 



