1892.] on the Motion of the Ether near the Earth. 571 



successive disturbances. A stationary telescope will look not at S, 

 but along M C to a point where the source was when it emitted the 

 wave M ; a moving telescope, if moving at same rate as source, 

 will look at S. Hence S M is sometimes called the apparent ray. 

 The angle S M C is the aberration 

 angle. ^i^- ^• 



Fig. 6 shows normal reflection 

 for the case of a moving source. 

 The mirror M reflects light re- 

 ceived from Si to a point S2, just 

 in time to catch the source there. 



Parenthetically I may say that 

 the time taken on the double 

 journey, S^ M 83, is not quite the 

 same as the double journey SMS, 

 when all is stationary, and that 

 this is the principle of Michelson's 

 great experiment. Normal reflection. 



For the rest of the lecture I am going to call the medium which 

 conveys light, " ether " simply. Every one knows that ether is the 

 light conveying medium, however little else they know about the 

 properties of that tremendously important material. 



We have arrived at this : that a uniform ether stream all through 

 space causes no aberration, no error in fixing direction. It blows 

 the waves along, but it does not disturb the line of vision. 



Stellar aberration exists, but it depends on motion of observer, 

 and on motion of observer only. Etherial motion has no effect upon 

 it, and when the observer is stationary with respect to object, as he 

 is when using a terrestrial telescope, there is no aberration at all. 



Surveying operations are not rendered the least inaccurate by 

 the existence of a universal ethereal drift ; and they therefore afford 

 no means of detecting it. 



But observe that everything depends on the ethereal motion being 

 uniform everywhere, inside as well as outside the telescope, and 

 along the whole path of the ray. If stationary anywhere it must 

 be stationary altogether. There must be no boundary betweeen 

 stationary and moving ether, no plane of slip, no quicker motion 

 even in some regions than in others. For (referring back to the 

 remarks preceding Fig. 4) if the ether in receiver is stagnant while 

 outside it is moving, a wave which has advanced and drifted as far as 

 the telescope will cease to drift as soon as it gets inside, but will 

 advance simply along the wave normal ; and in general at the 

 boundary of any such change of motion a ray will be bent, and an 

 observer looking along the ray will see the source not in its true 

 position, not even in the apparent position appropiate to his own 

 motion, but lagging behind that position. 



Such an aberration as this, a lag or negative aberration, has never 

 yet been observed ; but if there is any slip between layers of ether, 



2 Q 2 



