1404 
constant deviation, from which the prism had been removed, and 
replaced by a small totally reflecting prism. 
By placing a grating replica before the object glass of the photo- 
graphic camera, a spectrum of the source of light could be 
photographed. 
The most active part in this spectrum could be made directly 
visible by putting a wedge of smoked glass with the side horizontal 
before the slit of the collimator (KeNNern Mers method). 
The prismatic action of the wedge was counteracted by a second 
wedge of clear glass. 
The result for the effective wave-length 4 was 4750 A°, with an 
uncertainty of + 25 A°. | 
5. This accuracy is, however, sufficient. This can be verified by 
numerical calculation, or by the following consideration. 
We know (II, 9) that the optical effect is given by the formula: 
4 bw du 
Ra tE a ; 
1 dA 
By deriving from this the value of md we can see how great 
the influence is of an error in the determination of the effective 
wave-length on the calculated effect. 
Instead of 2 we introduce the frequency », which brings about a 
du 
slight simplication, and we put p=w—1-+pv ae Then: 
v 
1dA 1 dy. 1 ; 
Ad” p.v dv anaf 6) 
Now: 
dp du d'u 
zz yv ; 
dv dv dy? 
for which we may write in approximation : 
d d 
De 
dv dv 
because u depends almost linearly on v. 
Equation (2) then becomes: 
