ae 
W. Gibbs on the measurement of wave-lengths. 299 
* 
the ends of the different parabolic curves representing the 
successive groups. 
In Table I, the elements of the computation and the differ- 
ences between the wave-lengths, as actually measured and as 
computed by the formulas, are given. Column N gives the 
number of each element, W the weight, K the scale-number, 
4 the observed wave-length, and 4 the difference between the 
observed values of the wave-lengths and the values calculated 
by the constants given in Table II. It must be borne in mind 
that the sign + or — refers to the excess or deficiency of the 
observed over the calculated wave-lengths. e column é gives 
the probable error in the case of each group; it may also be 
considered as expressing the probable error in the determination 
of any wave-length by the method of comparison, for a par- 
ticular portion of the scale. 
TABLE I. 
K a A € N 
K a A € 
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