78 PSYCHOLOGICAL EFFECTS OF ALCOHOL. 



jective, and makes too large demands on the skill and cooperation of 

 the observer. Objective records are best furnished by photographing 

 the movements of the eye. 



Of the available photographic techniques, the kinematographic 

 Chinese- white method of Judd 1 is less adapted to showing time changes 

 in the eye-movements than Dodge's continuous records by reflection 

 from the cornea. The latter is the method which we used in these 

 experiments. For a complete description of its technique as well as 

 for a full discussion of its theory, we must refer to the original papers. 2 



THEORY OF RECORDING THE MOVEMENTS OF THE EYE BY PHOTOGRAPHING THE 

 MOVEMENT OF A REFLECTION FROM THE CORNEA. 



The theory of the corneal reflection method, briefly stated, is that a 

 virtual image from an eccentrically mounted convex spherical mirror 

 will appear to move in the direction of the latter' s rotation when the 

 axis of rotation lies behind the center of curvature. Within a small 

 error the surface of the normal healthy cornea is a convex spherical 

 surface. Its optical surface is as exact as the visual process which it 

 conditions. If the radius of the curvature of the cornea were infini- 

 tesimal, the apparent movement of the corneal reflection would equal 

 the sine of the arc of eye-movement, measured on a great circle of the 

 eyeball. If, on the other hand, the radius of the cornea were equal to 

 the radius of the eyeball, and the latter rotated on its center of curva- 

 ture, the corneal reflection would appear to remain stationary. As a 

 matter of fact, neither of the above suppositions is true, and the apparent 

 movement of a corneal reflection actually lies somewhere between zero 

 and the sine of the angular movement of the eyeball. Since the average 

 radius of curvature of the center of the cornea is 7.7 mm. and the dis- 

 tance from its apex to the center of rotation of the eye averages 13.5 

 mm., the apparent movement of a distant object reflected from near the 

 center of the cornea will be slightly less than one-half the actual dis- 

 placement of the apex of the cornea, but always in the same direction. 

 More accurately, under the above conditions, the apparent movement 



joe 7 7 P\ Q 



of the corneal reflection will be - - = 7^ of the actual movement 



lo. 5 13.5 



of the eye for small arcs (Dodge 2 ). 



REACTION TIME OF THE EYE. 



All adequate data concerning the latent time of the eye-reaction 

 show that it is relatively long. According to the most extensive photo- 

 graphic data hitherto collected, that of Diefendorf and Dodge, 3 the 

 normal average latency of the eye-reactions is about 200 cr. 



l J\idd, McAllister and Steele, Yale Psychological Studies, 1905, new series, 1, No. 1. 

 2 Dodge, An Experimental Study of Visual Fixation. Monogiaph Supp. of the Psychol. 

 Review, No. 35, 1907. 



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



