MOMENTARY CHARACTER OF VISUAL STIMULI 243 



Further, since the "exposure time" in ordinary vision is very 

 brief, the coming to pass of adequate stimulation will depend 

 upon the readiness with which the retina changes its state will 

 depend, in other words, upon the rapidity of the very first stage 

 of the "rise of sensation" (or its fall) for that part of the retina 

 involved and for the particular condition in which it happens 

 to be at the moment. If that much be granted, it is not hard 

 to see how the external conditions, antecedent and present, 

 implied in the terms "adaption," or "contrast" and "glare" 

 may induce a state of the retina which will affect the beginning 

 of the response to a given stimulus; and make that particular 

 duration of the fixational pause, to which the eye musculature 

 is by idiosyncrasy and habit adjusted, inadequate for an other- 

 wise supraliminal stimulus. A different rhythm of ocular move- 

 ment must then take place; or else eye movements which are 

 adequate under more favorable conditions will have to be re- 

 peated in order that successful integration of the retinal impres- 

 sions may come about. 



One or two informal observations, confirmed by the statements 

 of persons other than the writer, may not be out of place here. 

 The writer has noticed that certain technologically indefensible 

 conditions of lighting may be perfectly comfortable as long as 

 the eyes are not used for critical work. When reading is begun 

 under the same conditions, discomfort referred to the eyes is 

 felt at once or very soon. Again, eye discomfort sets in rather 

 early when the material read is unfamiliar and hence difficult. 

 Here the delay may be due to a lag in apperception. The 

 fatigue incident to an unaccustomed rhythm of movement is 

 well known in the case of one forcing himself to keep step in 

 walking with another who, from anatomic, physiologic or temper- 

 amental causes has a step much different in length or period 

 from one's own. 



Theoretically, then, it would seem as though the momentary 

 exposure method might throw light on certain questions related 

 to the hygiene of vision. 



A few remarks on the experimental aspect of this question 

 will not be out of place. The first requisite is the control and 



