io8o VISION. 



is being gradually lowered, the blue will appear to become whiter and 

 the red blacker, and the difference in saturation is in Hering's opinion 

 the basis of the changes in apparent intensity. 



Most of the work on Purkinje's phenomenon has been done by direct 

 comparison of the brightness of different colours. Mace and Mcati used 

 the acuity method of colour photometry, and Haycraft has recently 

 investigated Purkinje's phenomenon for coloured papers by the flicker 

 and distance methods. The distance method gave curves for the 

 different colours which agreed very closely with those obtained by the 

 flicker method. One point about which there is difference of opinion 

 concerns the upper limit of Purkinje's phenomenon, and it is also 

 doubtful whether the relative brightness of different parts of the spec- 

 trum varies to any appreciable extent at ordinary luminosities. This 

 and other questions require investigation with spectral colours, and with 

 one of the objective methods of colour photometry; probably flicker 

 photometry would give the most satisfactory results. 



We may now return to consider whether Purkinje's phenomenon is 

 a function of intensity or of adaptation. It is generally agreed that 

 adaptation of the eye to the dark is a necessary condition for the 

 favourable observation of the phenomenon, but, according to Hering, 1 it 

 is the essential factor. He varied the two conditions independently of 

 one another, using two adjoining rooms separated by a partition, 

 with round holes filled with coloured glass. By altering the illumina- 

 tion of a screen in one room, the intensity of the coloured patches 

 could be varied, while by alteration of the illumination of the other 

 room the eyes of the observer in this room could be brought to any 

 degree of adaptation. It was found that if the eye remained in a 

 condition of light-adaptation, red and blue when lowered in intensity 

 became gradually blacker without change in relative brightness. Of 

 the two, the red retained its colour slightly the longer, and appeared 

 slightly brighter. On the other hand, adaptation of the eye to the dark 

 brings about the phenomenon, without any change in the actual illumina- 

 tion of the patches themselves, the blue becoming whiter and brighter than 

 the red. Hering points out that, in addition to the ordinary adaptation 

 on prolonged stay in the dark, a momentary adaptation occurs on lower- 

 ing the illumination, and this also produces the Purkinje phenomenon, 

 though in a less marked degree than with prolonged adaptation. 



Weber's law. The relation of the difference of stimulus, which 

 produces a just perceptible difference of sensation, to the total stimulus, 

 has been frequently investigated for vision. The method most widely 

 employed has been to observe the point at which a shadow illuminated 

 by one light just disappears (or becomes visible) against a back- 

 ground illuminated by two lights. In 1760, Bouguer found by this 

 method that the difference was independent of the absolute intensity. 

 Another method is to use Masson's disc, a white disc on which there is 

 a narrow black band, which gives a grey ring on rotation. The breadth 

 of the band is found, which gives a just perceptible ring with different 

 illuminations. The results of star photometry have also been used, on 

 the supposition that the differences between the different magnitudes 

 represent equal differences of sensation, but this method does not appear 

 to be satisfactory. The difference of stimulus necessary for a just 

 perceptible difference of sensation presents great variations, which 



1 Arch.f. d. ges. Physiol., Bonn, 1895, Bd. Ix. S. 519. 



