240 PERCY W. COBB 



eyes are rarely more than momentarily at rest. 2 Certainly is 

 this true of such work as is commonly accounted as " trying' ' 

 to the eyes. 



It is due to Troland, 3 starting from Bering's viewpoint, that 

 the development of a new phase of the theory of vision has been 

 forwarded, hi which the change of state of the retinal elements 

 is given first importance, and the conception of the (so to speak) 

 one to one representation of stimulus-brightness in retinal re- 

 sponse has been pushed into the background. Upon this hy- 

 pothesis, granted that a stimulus of constant intensity continues 

 to fall upon the same retinal area, the physiological condition 

 in that area tends toward a certain state of equilibrium and 

 approaches that state at a rate greater according to its remoteness 

 from that state, somewhat after the manner in which the temper- 

 ature of a body tends toward the temperature of the medium 

 in which it either takes on or loses heat. Removed to a medium 

 of different temperature, the body will at once tend toward the 

 new temperature, by the same law, at a rate proportional to the 

 temperature difference. 



Similarly with the retina. The temperature of the medium 

 represents that particular state of physico-chemical equilibrium 

 in the retina which would hypothetically be induced by the 

 continued action of the constant stimulus; a terminal state which 

 the retina can, under constant conditions, be conceived to ap- 

 proach indefinitely without reaching it in any definite time. A 

 new stimulus acts, and the retina begins a change of state, 

 initially from .whatever state it is in, toward a new terminal 

 state; the initial change being the more rapid the more remote 

 the new terminal state and vice versa. 



2 According to Huey (The Psychology and Pedagogy of Reading, New York, 

 1908) the movements executed by the eyes in reading occupy 0.04 to 0.06 second, 

 while the average duration of the fixational pause he finds to be 0.19 second in 

 one case, 0.11 in another. Dodge (Zeitsch. f. Psychol., lii, p. 373) states that 

 the shortest adequate pauses photographically recorded in one of his experiments 

 are 0.070 to 0.100 second. 



3 For a full presentation of this conception see: Troland, Am. Jour. Psychol., 

 1914, xxv, pp. 500-27; and further: Trans. Ilium. Eng. Soc., New York, 1916, 

 no. 9, pp. 947-66, and Jour. Opt. Soc. Am., i, 1917, pp. 3-15. 



