718 Relative Motion of Earth and Atther. 
the clockwise pencil, if the experiment were tried in the 
Northern hemisphere. 
The observation of interference-fringes produced by pencils 
which traversed a path 60x20 metres* presented so little 
difficulty, that it seems quite feasible to proceed to much 
ereater distances. 
In the case considered, the length of the path would be 
four kilometres. [If this length were doubled the area 
enclosed would be quadrupled and the expected displacement 
would be 2°8 fringes. 
A difficulty in the measurement of this displacement lies 
in the fact that it cannot be reversed (as was the case in the 
experiment + where the entire apparatus was rotated). A 
fiducial mark is, however, furnished by the image produced 
by one of the two pencils. 
Thus let ight, starting from a slit and rendered parallel by 
a collimator, fall on a glass plate the upper half of which is 
heavily silvered while the lower half is clear or lightly 
silvered. 
The light transmitted by the lower half is reflected round 
the circuit, returning to the glass plate through which it 
passes to the observing telescope—while the reflected part 
traverses the circuit in the opposite sense, returning to the 
glass plate where it is reflected to the observing telescope, 
interfering with the former pencil. 
Observing by reflexion from the upper half, the image of 
the slit is seen, and the cross-hair of the eyepiece is made to 
divide this image symmetrically. The upper half is now 
covered and the lower half clear. 
The system of interference-fringes should have its central 
or achromatic fringe bisected by the cross-hair if the ether 
rotates with the earth. If the zether does not partake of the 
earth’s motion of rotation, the central fringe will be displaced 
from the cross-hair by the amount calculated from the 
formula. 
A control is furnished by introducing another pair of 
mirrors in the path so as to make the area of the circuit so 
much smaller that the displacement would be negligible. 
The attempt to apply the same principle to the revolution 
of the earth about the sun is less promising. The formula 
for the displacement from noon to midnight is in this case 
Av 
4 : 
Fh acne RV “2 d cos 6, 
where A, A, d, and V have the same meaning as before, and 
* Am. Journ. Sci. vol. iii. 1897. + Phil. Mag. Dec. 1887. 
