[turnbull] a subjective PHENOMENON OF VISION 19 



The image does not, I think, follow a simple harmonic motion, 

 and herein probably lies the failure with the above apparatus. 



The Periodicity of the Vihj-ations and the Conditions Affecting it. 



34. For the most part the experiments have been confined to the 

 periodicity and the conditions affecting it. So far as the experiments 

 have now gone the periodicity is probably affected: — by the wave length 

 of homogeneous light impinging on the Fovea ; and by mixed light fall- 

 ing on the Fovea; and more certainly, by the intensity of the light; 

 the act of " fixing " the background ; and by the condition and relative 

 fatigue of the eyes and body (see Supplement, June, 1904). 



35. These factors and their relative importance may be studied 

 from the curves presented with this paper, and from the tabulated 

 results first given. The method of taking observations was as follows : — 

 (1) with the stop-watch, as a rule, the key, breaking the circuit was 

 pressed at the beginning and end of 10 counted oscillations (by one 

 oscillation throughout is meant a. complete to and fro motion) a pause 

 was then made allowing the assistant to read the watch and replace 

 the drop ; another 10 oscillations were then counted, another pause 

 made, and so on. (2) ^Vith the chronograph, the key was first pressed 

 when the eyes were " fixed," la second time when beginning to count, 

 a third time at the end of 5 oscillations, a fourth time at the end of 

 10, a fifth at the end of 15, and so on, no pauses being made during 

 the run. 



36. Although little can be learned from the desultory experiments, 

 made between January 5th and July 15th, 1895, the general results 

 of these were as follows : — Periodicity, apparently ,i unaffected by count- 

 ing with one eye open and one shut, no matter which eye was open; 

 also unaffected by bodily exercise, but affected undoubtedly (made some- 

 what greater) by continued use of eyes. 



37. The next observations (made from July 15th to September 

 1st, 1895) are, perhaps, a little more valuable, although poor methods 

 were used (and not sufficient rest was taken between the readinofs to 



' I say " apparently," because ot enough readings were taken to establish 

 a result. 



