190 
brass scale beneath, and on the sleeve was a fiducial line, in the form of a 
fine black wire. Three independent settings were made on each nodal point, 
the mean being taken as the position of the node. Since the figures were 
formed at temperatures different from those at which they were measured, 
the corresponding corrections for the expansion and contraction of the 
glass tubes were applied. 
Fig. 3. 
From such a series of measured inter-nodal distances the most proba- 
ble value of the wave length was calculated from the formula, 
(n—1) (a, —a,) + (n— 8) (An —a,) 
n (n® —1) 
Le eo 
where is the number of settings, and a,-a,-a, are the respective settings. 
The writer is indebted to Mr. P. P. Koch for a complete calibration of 
the brass scale used, in terms of the standard meter bar belonging to the 
Institute. Corresponding corrections have been computed and applied to 
all the measurements of both series. 
