30 ROYAL SOCIETY OF CANADA 



8. The rapidity of tlie oscillations is affected by four things, 

 viz.: — ^The colour of the light entering the eyes; the intensity of the 

 light; the distance of the focussed object viewed; and the effort of 

 maintaining a focus so as to count the oscillations. 



9. The blue end of the spectrum gives quicker oscillations than 

 the red end. 



A more intense light (within moderate limits) gives quicker oscil- 

 lations than a lesser light. 



A near object focussed gives quicker oscillations than a distant 

 object. 



And the effort of continuous " fixing " gives varying oscillations, 

 now faster, now slower, with a gradual slowing after long continual 

 effort. 



In order to have numerical comparisons the same series alone can 

 be used and average figures would be al)Out as follows, in times for a 

 single oscillation (to and fro) : — 



Blue light about -48 sees. 



Eed light " -53 



Near dark object " -544 " 



Distant dark object " -595 " 



Eyes exposed to darkness for long time. . . -700 " 



10. Regarding the explanation of this curious phenomenon it is 

 difficult to be sure, but the one that the writer leans to and considers 

 the most plausible is this : — 



The several layers of the crystalline lens of the eye are in; con- 

 tinuous motions of an oscillatory character, in most persons' eyes this 

 motion is so confused that the well known phenomenon called " light 

 dust" is the only thing observed, but in the writer's eyes, probably on 

 account of a little extra phosphorescence of the crystalline layers, or on 

 account of the imusual keenness of vision the oscillations, by long prac- 

 tice have become so distinct that they may be counted. 



With this assumption, that the motions of the crystalline layers 

 (one or more) are visible to the writer, the variations in the periodicities 

 are accounted for as follows : — 



The oscillations for blue light are quicker than those for red 

 because the eyes of young people are more sensitive to blue light and 

 less to red than older people (Langle/s Researches, Trans. Amer. Journ. 

 Sci., November, 1888), and consequently the reflex action of the ciliary 

 muscle acts more strongly in the case of blue light. 



Then the oscillations for a near focus are quicker because the lens 

 is more convex than for a distant object under the control again of the 



