VELOCITY OF WHITE AHD OE COLOUHED LIGHT. 
255 
In our case t 2 —t 1 =2 seconds. Hence we have 
&> l+ g 3 
2 
corresponding to v— 
i 
- • 
To determine the value of f we notice above that 
s 2~ s i =u {h~h) ~ if(h 2 h 2 ) 
so 
S 3 S -2— U {h ^ 3 ) if(t 3 2 ^ 2 ) 
(where s 3 is the reading of the clock mark two seconds later than the mark indicated 
by s 2 ), but t 2 —t 1 =t 3 —t 2 =2 seconds. Therefore, subtracting, we have 
whence 
( 5 3 —6 b) { s 2~ s i)—J{h h) 
=4/ 
(k, s 3 ) (% -?i) 
4 
It follows then that f is equal to the second alternate difference in the above 
example divided by 4. 
This value of f thus determined, is never perfectly constant; but we can interpolate 
so as to find its mean value in the interval between the reading \f and the 
reading corresponding to the signal given by the observer. 
| N.B .—-When through imperfection of adjustment the values of f were very 
discordant, the observation was always rejected.] 
To deduce the velocity v of rotation of the cylinder when the reading (corresponding 
to the signal) was s , we notice that 
v 2 =v 2 —2/(5— s) 
The first thing to be done in applying these formulae is to determine the value of f 
which is to be adopted. The record given above shows us that, at the reading 23,072, 
4/=329 ; and at the reading 32,309, 4/= 295 ; and we obtain for the value of 4f, at 
the signal reading 24,108, by simple proportion, 
4/ 1= 329-||x34 = 325. 
