[turnbull] a subjective PHENOMENON OF VISION 



23 



no case did G commence a set, — however, in the experiments, the re- 

 sults of which follow, considerable rests were always taken between 

 exposure of ej^es to differently coloured light, so that all colours were 

 treated alike. 



45. It may be further mentioned here that the periodicity for all 

 colours is usually a trifle greater at the end than at the beginning 

 of the day: but in all cases the difference is exceedingly small, and 

 the oscillations are, under like conditions, remarkable for their con- 

 stancy. 



46. To ascertain the periodicities obtaining during the " contest 

 of colours" ^ an I B glass and an E, glass was held before the eA'es with 

 a perpendicular partition between them touching the nose. An object 

 m the distance was " fixed '' and as the field alternated from E to I B, 

 and from T B to E, the oscillations were counted for one colour alone, 

 as it confused the assistant to record both changes and periodicities. 

 For example, during one set each time the field became red 5 oscil- 

 lations were counted, and in the next set each time the field became 

 indigo 5 oscillations were counted; the glasses for successive sets were 

 changed about for the two eyes; the data are given in the following 

 table : — 



* See Helmholtz " Physiologische Optik," etc. 



