240 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
adding these we have 
and 
also 
Thus 
1 + r + 2 -S=4 1 - (, ' +2)+ ? 
l-(r+2)+®;=pjl+* 
n 
N- 
B_ 
2-^n"-n')= p 
hi"—n' s 
N 
B 
2N B -g(n"-n') 
^ n" — n' 
, 2N b 
9+P-^r 
y_ 2m(n'-\-n")D B \ 
r+1 j 
8'(n"-n') 
B. 
2N 
+ • 
B 
■} 
when the 13th and 14th equalities are used. 
Description of the apparatus. 
The optical arrangements devised by M. Fizeau were admirably suited to the 
purpose, and were adopted, with modifications, by M. Cornu. 
The telescope of emission, which is also the observing telescope, is pointed towards 
the distant stations. At its focus is placed the revolving toothed wheel; between 
that and the eye-piece is a diagonally-inclined piece of unsilvered glass. 
The reflector designed by Fizeau for the distant station may be called a reflecting 
collimator; it consists of a telescope pointing towards the observing telescope, but, 
instead of having an eyepiece, it has in its principal focus a silver reflector. The 
advantages of this arrangement are its extreme simplicity and the facility of directing 
it—great accuracy of adjustment being unnecessary. 
The rays of light coming from the sun, or any source of light," are concentrated by 
means of a lens to throw an image of the source of light, after reflection at the 
diagonal mirror, upon the edge of the toothed wheel. If a tooth be not in the way 
these rays spread out and fill the object-glass, whence they proceed to the distant 
reflecting collimator, where they arrive all nearly parallel in direction. There they 
are caused by the object-glass to throw upon the focal mirror a luminous image 
of the object-glass of the observing telescope. The rays are then reflected to the 
object-glass, whence they proceed to the observing telescope and produce at its focus 
an image of the object-glass of the reflecting collimator. If no tooth be in the way, 
* In the experiments of 1880-81 we always used the electric lig'ht. 
