576 Professor Oliver Lodge [April 1, 



alike ; he looked at the deviated image of a star, not at its dispersed 

 image or spectrum, else he might have detected the change-of- 

 frequency-effect due to motion of source or receiver first actually seen 

 by Dr. Huggins. I do not think he would have seen it, because I do 

 not suppose his arrangements were delicate enough for that very small 

 effect ; but there is no error in the conception of his experiment, as 

 Prof. Mascart has inadvertently suggested there was. 



Then Maxwell repeated the attempt in a much more powerful 

 manner, a method which could have detected a very minute effect 

 indeed, and Mascart has also repeated it in a simple form. All are 

 absolutely negative. 



Well, what about aberration? If one looks through a moving 

 stratum, say a spinning glass disk, there ought to be a shift caused 

 by the motion (see Fig. 4). The experiment has not been tried, but 

 I entertain no doubt about its result, though a high speed and con- 

 siderable thickness of glass or other medium is necessary to produce 

 even a microscopic apparent displacement of objects seen through it. 



But the speed of the earth is available, and the whole length of a 

 telescope tube may be filled with water ; surely that is enough to 

 displace rays of light appreciably. 



Sir Geo. Airy tried it at Greenwich on a star, with an appropriate 

 zenith-sector full of water. Stars were seen through the water- 

 telescope precisely as through an air telescope. A negative result 

 again. 



Stellar observations, however, are unnecessarily difficult. Fresnel 

 had said that a terrestrial source of light would do just as well. He 

 had also (being a man of exceeding genius) predicted that nothing 

 would happen. Hoek has now tried it in a perfect manner and nothing 

 did happen. 



Since then Prof. Mascart w^ith great pertinacity has attacked the 

 phenomena of thick plates, Newton's rings, double refraction, and the 

 rotatory phenomenon of quartz ; but he has found absolutely nothing 

 attributable to a stream of ether past the earth. 



The only positive result ever supposed to be attained was in a 

 very difficult polarisation observation by Fizeau in 1859. As this 

 has not yet been repeated, it is safest at present to ignore it, though 

 by no means to forget that it wants repeating. 



Fizeau also suggested, but did not attempt, what seems an easier 

 experiment, with fore and aft thermopiles and a source between them, 

 to observe the drift of a medium by its convection of energy ; but 

 arguments based on the law of exchanges* tend to show, and do show 

 as I think, that a probable alteration of radiating power due to motion 

 through a medium would just compensate the effect otherwise to be 

 expected. 



We may summarise most of these statements as follows : — 



* Lord Kayleigb, 'Nature,' March 25, 1892. 



