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.

**Stansfield** (Phil.

(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.