Abstract
This paper first outlines the problems encountered in the photometry of coloured light sources, in particular as they are associated with the colour-characteristics of the eye. The optimum conditions for such work are shortly discussed. In the light of these findings the various experimental determinations of the mean relative luminosity function are examined seriatim under two headings—first, the investigations forming the basis of the agreed Standard Curve, and, second, more recent measurements differing somewhat from the older ones. Possible reasons or the observed differences are enumerated and examined.
The author concludes that there are at present no certain grounds for disputing the acceptability of the Standard Curve, and supports this conclusion with evidence derived from general photometric experience. A final section of the paper deals with the peculiar difficulties encountered in heterochromatic photometry at low brightness-levels, where the colour sensitivity of the eye varies greatly with the level used.