MOMENTARY CHARACTER OF VISUAL STIMULI 241 



But with the saltatory behavior of fixation, which has been 

 well demonstrated by photographic records of the eye-movements 

 executed in reading, no part of the retina or at least not all 

 parts critical in recognizing a given detail can be conceived to 

 reach a state of equilibrium, unless by way of a rare exception. 

 The rule will be that a given retinal element only begins to make 

 a change toward a certain state, when the process is cut short 

 by a shift of fixation, before a state of equilibrium is even 

 approximated. 



Now what happens when, as in most experiments on visual 

 thresholds, a part of the retina of a fixated eye is subjected to 

 stimulation for one-half or one or several seconds? And what 

 bearing has this on what happens to the retina in the case in 

 which such a stimulus is cut off in a few thousandths or hun- 

 dreths of a second? Obviously the two occurrences bear a certain 

 mutual relation, but the latter represents the initiation of a 

 change while the former represents more nearly the final stages 

 of the same sort of change; the former is a type of stimulation 

 whose result must be conceived to depend more largely upon 

 the terminal state corresponding to the continued action of the 

 stimulus, while the latter will give a result depending upon the 

 quickness of the retina to respond, a result which, I believe, is 

 more truly representative of the behavior of the sense-organ in 

 the course of its every-day performance. 



By way of thesis, for experimental verification or refutation, 

 the following may be laid down : 



1. Every-day vision involves, as a rule, only momentary fix- 

 ation, during which most of the details of the total impression 

 are taken in, not by the fovea, but by the parafoveal retina. 

 Possibly the peripheral retina is best adapted to momentary 

 stimulation. At least it may be said that the peripheral retina 

 is highly sensitive to change of stimulus; and also to movement, 

 which, when analysed as a stimulus, resolves itself into changes 

 of stimulation of retinal elements. 



2. The adequate stimuli to such vision are a priori by no 

 means identical with those adequate to the foveal retina, espe- 

 cially when in the latter case the time of exposure to the stimulus 



