Full Idea
For positivists it was taken that while theory change meant change in the meaning of theoretical terms, the meaning of observational terms was invariant under theory change.
Gist of Idea
Positivists hold that theoretical terms change, but observation terms don't
Source
W.H. Newton-Smith (The Rationality of Science [1981], I.4)
Book Reference
Newton-Smith,W.H.: 'The Rationality of Science' [RKP 1981], p.11