Eye Movement. 79 



EYE MOVEMENTS. 



The general apparatus employed in this measurement was the same 

 as that for the eye reactions, and these two tests followed each other in 

 quick succession. There was not the slightest change in the technique 

 from the methods used in the previous research. On a black field two 

 white dots, separated by 40° on the arc of vision, served as the fixation 

 points. The falling of a shutter which allowed the light to fall upon 

 the eye revealed, at the same moment, the white dot at the right. The 

 appearance of this dot was the signal to the subject to begin the series 

 of movements. The use of such simple fixation marks has the disad- 

 vantage that as there is no practical need for looking accurately at 

 them, as one would do in reading, the subject after some practice may 

 become negligent and slight the fixation of them, thus decreasing the 

 amplitude of his eye movements. 1 



The tendency to disregard accuracy of fixation of the marks is clearly 

 indicated in the photographic record, for, as Dodge has pointed out, the 

 eye, in looking from one mark to another which is a considerable dis- 

 tance away, accomplishes this by one long "saccadic" 2 movement, fol- 

 lowed by a small readjustment, which we may term corrective move- 

 ment. These corrective movements tend to become not only smaller 

 in size but fewer in number and finally to disappear from successive 

 records as the subject becomes more and more used to the measure- 

 ment. There must be continual emphasis laid upon the necessity for 

 the subject to look accurately at the fixation mark, and stress of this 

 point by the operator may not always be successful in holding the sub- 

 ject to careful work. Unfortunately, not realizing adequately the 

 extent of this tendency, we placed speed before accuracy in our instruc- 

 tions, whereas the emphasis should have been reversed. It is probable 

 that speed should not have been mentioned with this subject, since he 

 not only neglected careful fixation of the marks, but at times seemed to 

 disregard them entirely, with the result that the eye-movements were 

 of very irregular amplitude, mostly far short of the desired 40°. 



To make clear the difficulty in dealing with these records and the 

 impossibility of presenting the data in exactly the same form as that 

 given by previous authors, 7 eye-movement records have been placed 

 together in figure 8. The records, which were taken on photographic 

 plates, 2\ by 7 inches, are to be read from the bottom upward. The 

 beginning of the first vertical line represents the moment when the 

 light was turned on the eye by the falling shutter, which at the same 



1 A technique used by Diefendorf and Dodge (see Brain, 1908, p. 458) would doubtless meet 

 this difficulty. They exposed in succession isolated numerals in two different areas of the field 

 about 25° apart. The reading of one numeral by the patient was the signal for the operator to 

 cover it and to expose another on the opposite side of the field. This provided a good and suffi- 

 cient reason for looking carefully at the marks exposed and made the eye movements between 

 merely an incident which the subject performed without any thought of its being the important 

 feature of the measurement. 



2 Dodge, Psychol. Bull., 1916, 13, p. 422. 



