[ALLEN] PHENOMENA OF THE PERSISTENCE OF VISION 



201 



and greatest in the yellowish green. This latter colour also most rapidly 

 bleaches the visual purple. The region of the spectTum, therefore, which 

 is most absorbed by the visual purple coincides with the elevation of the 

 persistency curves. This absorption of light is therefore the ver}^ pro- 

 bable cause of the elevations. 



IV. In the paper to which reference has been made an experiment 

 was described, i in which a persistency curve was obtained when the eye 

 was fatigued with, white light from an arc light. This curve and its 



Fig. 4. 



comparison normal curve are shown in figure 5, i which is taken from the 

 paper previously cited. 



As white light, indistinguishjable | from ordinary white, may be ob- 

 tained by mixing two complementary colours, observations of the per- 

 sistence of vision were made after fatiguing the retina with proper com- 

 binia-tions of colours. Two overlapping spectra were formed with a 

 Helmholtz colour mixing spectrometer, -and the selected complemen- 

 taries combined in la shutter eye-piece. By means of Xicol prisms in 

 the doUimators, the intensities of the i two spectra were adjusted until 

 the narrow field of light in the eye-piece was white. The first comple- 

 mentary colours used were yellow {\ = .bT7 fi) and blue {X = .474 /i) 

 as determined by Von Kries. The readings are given in table 4, 

 with the corresponding normal values, and -are shown graphically in 



