The theory of transmission oximetry is extended to take into account the scattering of light by whole blood, and conditions are formulated under which a single-scale oximeter may be constructed. An instrument is described which has been designed particularly for cardiac catheterization procedures. After setting up on an initial sample of blood, a series of readings can be taken in rapid succession without the need to manipulate controls.

Equipment based on a photoelectric detection method is described which has been designed for counting apple cells (approximate size distribution 70-500 μ length) but which may be used with any reasonably opaque particles of suitable size which can be held in suspension in a clear low-viscosity liquid. The accuracy of a single run is about ±3% including sampling errors. This error may be reduced by averaging several samples.

A circuit for controlling the r.m.s. value of an a.c. signal is analysed and practical results are discussed. The speed of response is such that for rates of change of 2.5% per second the output is controlled continuously within 1%. An alternative arrangement for maintaining, at a preset level, the peak value of repetitive pulses is also given.