One of the neatest applications of this principle is that described by Edser and Stansfield (Phil.
Stansfield 4 and L.
5 According to Stansfield there are three companion lines on either side of a central line, which consists of two lines of unequal brightness.
(14) which is equivalent to the form given by Stansfield, but with the neutral temperature To explicitly included.
In many cases a formula of the last type would be quite inapplicable, as Stansfield remarks, but the difference between the three is often much less than might be supposed.
Pt.-Pt.- couple, if we calculate three formulae of the above types to satisfy the same pair of observations at ° -445° and 0°-z000° C., we shall find that the formula s =constant lies midway between that of Tait and that of Stansfield, but the difference between the formulae is of the same order as that between different observers.
The writer's observations agree more nearly with the assumption s=constant, those of Stansfield with s=c/T.