322 0. A. Young—Displacement of lines in the 
be affected by any relative motion of approach or recession 
between it and the source of the vibrations. 
It is not difficult to verify this conclusion in the case of 
sound waves. The beautiful experiments of Vogel, lately 
published, show as the result of careful quantitative measure- 
ments, that the pitch of a locomotive whistle actually undergoes 
the precise alterations which theory requires, when the engi 
is either approaching the observer, or receding from him at a 
known velocity. 
Undoubtedly a considerable point would be gained if we 
could obtain a similar verification in «the case of light—if an 
alteration in the luminous pitch or wave length, produced in a 
ray of light by some known rate of motion, could actually be 
made sensible, measured, and shown to coincide with theory 
within observational limits of error. This verification unfor 
tunately is not easy to obtain, because the velocity of light 1s 
so enormous that it is difficult to find an object sufficiently 
bright, and moving rapidly enough, to make the change of 
wave length perceptible in our instruments. 2 
I think it was Zollner who first suggested that the rotation 
of the sun might furnish the desired test, since its eastern an 
western limbs have a relative motion of nearly 24 English miles 
er second along the line of sight. But the displacement 0 
ines in the spectriim due to this velocity is so small (in the 
case of the D lines about ,'; of the distance between them) that 
the dispersive power of the instruments heretolore employed by 
most observers, has been insufficient to make it clearly evident. 
Vogel alone (in 1871) seems to have succeeded in getting any 
measurements; his results for the sun’s equatorial velocity of 
rotation ranging from 0°35 to 0°42 of a geographical mile, oF 
from 1°62 to 1°94 English miles. 
By using a diffraction grating, however, combined with 4 
prism in such a way as to separate the overlapping spectra i 
the higher orders from each other, as described in my rece? 
note* on the duplicity of the 1474 line, it is possible to oe 
much greater dispersive power, and the displacement then 
comes quite sensible. ; 
The apparatus which I have employed consisted of oe 
fine diffraction grating of 8,640 lines to the inch (for whic 
am indebted to Mr. Rutherfard) combined with a telescoh” 
: : he refracting edg® 
and the object glass of the eye-telescope. The ap on the grat 
en 
ing. The grating, collimator, &c., were mounted on * 4 to be 
framework constructed for the purpose, and arrang 
* This Journal, June, 1876. 
