500 



NATURE 



[September 22, 1892 



sumably 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 19 miles a second. Send a beam of light 

 through it one way and it will be hurried ; its velocity, instead 



//Si 



Fig. 7.— Plan of interference kaleidoscope. 



of being 140,000 miles a second, will be 140,009 miles. Send a 

 beam of light the other way, and its velocity will be I39>99i '■> 

 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 inter- 

 ference 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 experi- 

 ment 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, " doubly negative," and we require some guarantee that 

 the condition was right for seeing what might really have been 

 in some sort there. Hence Mascart and Jamin's modification 

 of the experiment is preferable (Fig. 9). The thing now looked 

 for is a shift of already existing interference bands, when the 



above apparatus is turned so as to have different aspects with, 

 respect to the earth's motion ; but no shift was seen. 



Interference methods all fail to display any trace of relative 

 motion between earth and ether. 



Try other phenomena then. Try refraction. The index of 

 refraction of glass is known to depend on the ratio of the speed 

 of light outside, to the speed inside, the glass. If then the ether 

 be streaming through glass, the velocity of light will be different 

 inside it according as it travels with the stream or against it,. 

 and so the index of refraction will be different. Arago was the 

 first to try this experiment, by placing an achromatic prism in 

 front of a telescope on a mural circle, and observing the devia- 

 tion it produced on stars. 



Observe that it was an achromatic prism, treating all wave- 

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

 its dispeised 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 iis result, though a 

 high speed and considerable thickness of glass or other medium 

 is necessary to produce even a microscopic apparent displace- 

 ment 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 appro- 

 priate 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) pre- 

 dicted that nothing would happen. Hoek has now tried it in. 

 a perfect manner and nothing did happen. 



Since then Prof. Mascart with great pertinacity has attacked 

 the phenomena of thick plates, Newton's rings, double re- 

 fraction, 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 w as in a 

 very difficult polarization 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 diift of a medium by its convec- 

 tion 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 summarize most of these statements as follows : — 



Summary, 



A real and apparent change of wave - 

 ; length. 



A real but not apparent error in. 

 direction. 



No lag of phase or change of inten- 

 sity, except that appropriate to 

 altered wave-length. 



No change of frequency. 



No error in direction. 



A real lag of phase, but undetectable 

 without control over the medium. 



A change of intensity corresponding 

 to different distance, but compen- 

 sated by change of radiating power. 



Source alone 

 produces ... 



moving 



Medium alone moving, 

 or source and receiver 

 moving together, pro- 

 duces 



Lord Rayleigh (NaT'.re, March 25, i?q2). 



NO. I 195, VOL. 46] 



