I ision of Colours of Varying Intensity. 85 



rare diminished. There is a marked flattening of the curves 

 :as they approach the lowest, which is that for the dimmest 

 spectrum. 



Fiff. 3. 



•40/z. -45 



•55 -60 



Wave Length. 



A somewhat similar group of three luminosity curves was 

 obtained by Haycraft* by the flicker method, who, however, 

 employed colour disks at three different intensities of illumi- 

 nation instead of spectral colours. His curves are somewhat 

 the same as those in fig. 3, but are noticeable for a much 

 greater displacement towards the blue of the brightest point 

 than is indicated in my own curves. 



In Table II. the first column shows the angles, «, between 

 the principal sections of the nicols, the second the values 

 of cos 2 a to which the luminosities are proportional, and 

 the remaining columns in pairs under the wave-lengths, 

 X'725//,, &c, show the values of the persistence of vision, D, 

 -and their reciprocals. 



* Journ. of Physiol, vol. xxi. (1897). 



