242 PERCY W. COBB 



is one of longer duration, as in the practical vision of a barely- 

 visible object. 



3. The condition of eye fatigue and eye strain are to besought, 

 not hi relation to the unusual and more or less episodal perform- 

 ances of the eyes; but rather in relation to such performances 

 of the visual organs as are the rule in the course of their ordinary 

 use. 



In detail, such performance of the eyes as is the rule hi typical 

 eye-work, as hi reading for example, consists in successive mo- 

 mentary fixations, in which details of the words and of the 

 letters forming them are impressed chiefly upon the parafoveal 

 retina; and in sudden leaps from one point of fixation to the 

 next, during which it seems probable that no neural impulse 

 originating in an impression upon the retina is able to take part 

 in the sensory integrative process. 4 



It is further to be noted in this connection, that the details 

 passed over in these fixational leaps are by no means left out of 

 the sensory integrative process. On the contrary, they appear 

 to be the very details upon which the characteristics of one 

 text depend, as distinguished from another. They are sensed 

 as a result of momentary stimulation and they are sensed not 

 through the foveal, but through the parafoveal retinal elements, 

 which, even with a relatively leisurely mode of stimulation have 

 been shown to be far behind the foveal elements hi point of visual 

 acuity. It would follow therefore that the stimuli which are 

 adequate for the ordinary process of reading are, for the con- 

 ditions under which they are seen, much nearer to the threshold 

 than they would appear to be from a measurement of their 

 visibility at the fovea made in the usual way with direct fixation 

 and more than momentary stimulation; and it would seem, 

 therefore, that a larger amount of our seeing is accomplished in 

 fact at or near to the threshold than would at first thought be 

 supposed. 



4 A conclusive piece of experimental work upon the retinal anesthesia which 

 takes place during voluntary eye-movement is that of Holt, Psychol. Rev. Monog. 

 Suppl., iv, pp. 3-47. 



