First of all there is discussed the best method of obtaining an intense beam of parallel rays having each ray of equal brightness, as the production of such a beam is a pre-requisite of all spectrophotometric measurements of light absorption. Next there are briefly described three new types of “flicker“ spectrophotometer, the first of these depending on polarisation, the second on refraction, and the third on reflection. All three are quite successful: which it is best to use depends on the circumstances of the case. Finally, three general governing principles are laid down which would seem to apply equally to all types of flicker photometer.

There remains the pleasant duty of expressing my thanks to the Trustees of the Carnegie Trust for the various grants which have made these somewhat expensive investigations possible.