After a brief historical survey of attempts at the realisation of a physical eye of defined spectral sensitivity, the earlier methods and technique employed are appraised. Subsequently, the problem is stated, both from the theoretical and practical aspects. The main treatment of the paper deals with the description of the multi-purpose apparatus which the authors have developed and which is suitable for use as an “average eye,” as defined by the Commission Internationale d'Eclairage. It can be adapted for use as a physical colorimeter indicating directly the C.I.E. trichromatic coefficients of coloured light and as an instrument for measuring spectral luminosity distribution in terms of the proportion of the total luminosity in each of 8 defined spectral bands. Finally, the possible general application to commercial photometry, colorimetry and colour rendering measurements, is discussed.