COMPLEX NEURAL ARCS. 77 



same for a longer interval, Dodge 1 has shown that physiological causes 

 are constantly operating to produce lapsed fixations which must be 

 corrected by frequent eye-reactions. If these primary and corrective 

 reactions have occurred on the average of once a second since birth, 

 the number of eye-reactions of our normal subjects before they reached 

 the psychological laboratory was enormous. In many uses of the eyes, as 

 in reading for example, eye-reactions occur on the average at more than 

 twice that rate. Obviously, the eye-reaction is one which the subject 

 does not have to learn arbitrarily for experimental purposes, like lifting 

 his finger in reaction to a noise or to the appearance of a predetermined 

 color. It is a natural part of his vital equipment a necessary pre- 

 condition of the effective visual apprehension of his environment. But, 

 as a rule, one is unconscious both of the exact stimulus to movement 

 and of the consequent reaction of the eye. Under such circumstances 

 there can be no distinguishable motor and sensory types. In view of 

 the importance of eye-reaction in normal life, in view of its pre-experi- 

 mental practice, and its independence of arbitrary voluntary inter- 

 ference and freedom from change of type, the eye-reaction entirely 

 satisfied our criteria of available processes for measurements. The fact 

 that Diefendorf and Dodge 2 secured comparable measurements of their 

 eye-reactions from a group of over 40 insane patients may be cited as 

 relevant evidence that the experiment does not make exorbitant 

 demands on the cooperation of the subject. Moreover, the elaboration 

 of the motor impulse that carries the eye to an approximately correct 

 position for its new fixation in a single sweep is as well understood as 

 any highly coordinated voluntary act. It has been subjected to an 

 extraordinary amount of psychological and physiological investigation. 

 It involves at least no arbitrarily changing factors. Finally, the photo- 

 graphic technique for recording the eye-reactions is simple, dependable, 

 and accurate. 



METHODS FOR RECORDING THE EYE-REACTIONS. 



We may pass over without discussion the earlier non-graphic method 

 of measuring the eye-reactions. The first reliable data on the reaction 

 of the eye was given by the blind-spot method. 3 It depended on 

 measuring the necessary duration of a light which fell within the blind 

 spot while the eye was at rest, but emerged from the blind spot on to a 

 sensitive part of the retina when the eye moved to see a suddenly 

 appearing peripheral object. If this light was seen its duration must 

 have been greater than the latent time of the eye. If it was not seen 

 its duration was less. Such an experimental method, however accurate, 

 would have been impractical in a study like the present. It is too sub- 



'Dodge, An Experimental Study of Visual Fixation. Monograph Supp. of the Psychol. 

 Review, No. 35, 1907. 



2 Diefendorf and Dodge, Brain, 1908, 31, p. 451. 

 3 Dodge, Psychol. Review, 1899, 6, p. 477. 



