1 078 VISION. 



A very simple allied method l depends on a phenomenon described by 

 Jastrow. If a rod be moved in front of a rotating disc on which a colour and 

 a grey are being mixed, coloured bands will be seen following the rod, except 

 with a certain grey in the case of each colour. The grey found to give no 

 bands with a given colour agrees very closely with that which ceases to nicker 

 at the same time as that colour. 



Another method differing in principle is to determine the distance at which 

 small areas of different colours become invisible. This has lately been used by 

 Haycraft 2 with results agreeing very closely with those of the flicker method. 



The dependence of the amount of contraction of the pupil on intensity is the 

 principle of another method, first advocated as a general photometric method by 

 Gorham. 3 The size of the pupil is observed entoptically, and this method has, 

 in the case of coloured papers, been found to give results closely agreeing with 

 the flicker method. 4 A different application of the method has been made by 

 Haycraft, 5 who measured the size of the pupil, when exposed to different parts 

 of the spectrum, on photograms obtained by the flash-light, and obtained a 

 curve of luminosity of the spectrum agreeing with that of flicker. 



Another method proposed by Martius 6 depends on the fact that a coloured 

 patch becomes darker on fixation when on a background darker than itself, 

 lighter when on a lighter ground. Accordingly, the intensity of a grey on 

 which a coloured patch appears to become neither lighter or darker may be 

 taken as a measure of the brightness of the colour. Martius found it more 

 convenient to use a grey patch on a coloured ground owing to absence of dis- 

 turbance by the complementary colour of the after-image, and the brightness 

 of a series of coloured papers was found by this method to agree fairly with the 

 results of Fraunhofer and Vierordt. The results, however, differ considerably 

 from those of other methods. 



The various methods which have been described give results which, 

 on the whole, agree with one another. Haycraft found a close agree- 

 ment between the flicker and distance methods, and the flicker, band, and 

 pupil methods also give concordant results, and agree approximately with 

 the results of direct comparison. The assumption that the attribute 

 which is being measured by these methods is brightness, rests partly on 

 the correspondence with direct comparison, but more on the fact that a 

 method, such as nicker, seems to be reliable as a means of determining 

 the brightness of different physical lights of the same colour, in which 

 case its results can be checked by the ordinary physical methods of 

 photometry. As it measures brightness in that case, we may conclude 

 that it also measures brightness in the case of lights of different colours, 

 although it is difficult to exclude the possibility that it may also be in 

 some degree affected by the wave-length. In the section on retinal per- 

 sistence and flicker, it was stated that probably flicker depends wholly 

 on intensity and not on wave-length, and this statement rests to a 

 great extent on the reliability of flicker as a photometric test. 



Purkinje's phenomenon. — One difficulty of colour photometry 

 is at the same time a phenomenon of great interest. The relative 

 brightness of different colours varies with the absolute intensity. 

 Purkinje observed that a red and a blue, which appeared equally bright 

 with ordinary illumination, differed in brightness at lower intensities, the 



1 Journ. Physiol., Cambridge and London, 1897, vol. xxii. p. 137. 



2 Ibid., vol. xxi. p. 126. 



3 Proc. Roy. Soc. London, 1884, vol. xxxvii. p. 425. 



4 Journ. Physiol., Cambridge and London, 1897, vol. xxii. p. 137. 

 6 Communication to Physiological Society, not yet published. 



6 "Beitr. z. Psych, u. Phil.," Leipzig, 1896, Bd. i. S. 95. 



