50 Dr. J. B. Haycraft. Luminosity and Photometry. 



light adaptation for strong stimuli, and passed gradually into the 

 condition of dark adaptation as the stimuli became feebler. Tiny 

 discs of coloured paper were pasted on a black background, and 

 the distance determined at which these spots could just be seen. 

 The room in which both the discs and the observer were placed 

 was illuminated by a graduated gas burner reading from twenty 

 candles downwards. For low luminosities when the quality of 

 the flame changes, the burner was raised towards the ceiling to 

 measurable distances. 



Order of potency of discs ; distance at which they were seen. 



At high illuminations Yellow, red, green, blue. 



„ low „ Yellow, green, red, blue. 



,, very low illuminations . . Green, yellow, blue, red. 



The potency of different portions of the spectrum was also deter- 

 mined by the method of flicker photometry. A semi-disc, rotated by 

 an electromotor between the source of light and the slit of the spec- 

 troscope, repeatedly cut off the light entering the instrument. The 

 rate of rotation of the semi-disc was varied by the use of a graduated 

 resistance, and the rate could be accurately determined. At a low 

 speed of rotation the spectrum flickered, except at its ends; the 

 * position of these two ends was determined. On increasing the speed 

 of rotation, the flickering centre was narrowed, and another two 

 points were obtained. From a number of such observations a curve, 

 giving the potency of the spectrum to produce flickering, was con- 

 structed. With a dim spectrum the green, near E, is the most 

 potent, and the violet half of the spectrum is more potent than the 

 red. On increasing the luminosity of the spectrum, the apex of the 

 curve shifts from the green into the yellow, and the red half becomes 

 more potent than the violet. Flicker curves from coloured papers 

 observed at different luminosities were also obtained. The graduated 

 gas-bnrner was here utilised, and throughout these experiments, 

 gas-light was used as the source of light. The curves obtained 

 by flicker photometry, and those obtained by the method of the 

 minimal effective stimulus, strikingly resemble each other. The 

 nature of the flicker effect is discussed, and the curves obtained by 

 the above methods are given in a paper shortly to appear in the 

 ' Journal of Physiology.' 



