and the Luminiferous Ether. 121 
which have thus far been obtained. In the same letter it was 
also stated that the reason why such measurements could not 
be made at the earth’s surface was that we have thus far no 
method for measuring the velocity of light which does not 
involve the necessity of returning the light over its path, 
whereby it would lose nearly as much as was gained in going. 
The difference depending on the square of the ratio of the 
two pb serapey according to Maxwell, is far too small to 
measu 
The follows is intended to show that, with a wave-length 
ellow light as a standard, the quantity—if it exists—is 
par Ae measurable. 
: D 
Using the same notation as before we have io, and 
oe The whole time occupied therefore in going and 
returning T+T, = If, however, the light had trav: 
eled in a direction at right angles to the earth’s motion it 
would be entirely unaffected and the time of going and return- 
ing would be, therefore, 25=2T,, The difference between the 
times T+T', and ci is 
oie, vw 
or nearly me, In the time ¢ the light would travel a dist- 
ance Ve=2VT,2,=2D05 
That is, the actual distance the light travels in the first case 
2 
is greater than in the second, by the quantity 2D 
ictisa only the velocity of se earth in orbit, the 
rati . So 
10 = 7 sae — approximately, and = 7" 100 000 000 If 
1200 millimeters, or in a . of este light, 2 000 000, 
tl a 
1en in terms of the same unit, aDy= rae 
If, therefore, an apparatus is so constructed as to permit two 
pencils of light, which have traveled over paths at right angles 
to each other, to interfere, the pencil which has traveled in the 
direction of the earth’s motion, will in reality travel ix of a 
wave-length farther than it would have done, were the earth at 
es he other pencil being at right angles to the motion 
would not be affected. 
