The authors discuss the effect of artificial light on colour rendition using the spectral energy curves of various light sources to illustrate the deficiencies in the red and blue/green regions of the spectrum of present daylight fluorescent tubes. They also describe experiments carried out by Dr. Crawford to determine the tolerance of the human eye to deficiences of this kind in the spectrum, by testing how much of any part of the spectrum of a light source could be removed without causing a noticable change in the coloursof objects illuminated by it.