Abstract
The importance of narrow slits in the spectrophotometric determination of extinction coefficients has been demonstrated. Care is needed in the B.P. 1958 spectrophotometric tests or assays of apomorphine hydrochloride, chloroquine phosphate, chloroquine sulphate, diiodohydroxyquinoline, naphazoline nitrate, papaverine hydrochloride and especially procyclidine hydrochloride, to avoid spuriously low results.