364 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



is exceedingly close. The importance of these facts in forming 

 conclusions as to the mechanism of visual processes will be pointed 

 out when we have considered a few more experimental observa- 

 tions regarding central and peripheral vision in the two phases of 

 adaptation. 



A set of experiments, apparently of little practical import- 

 ance, has proved very interesting with respect to the phenomena 

 of adaptation. These deal with the effects which follow the 

 application of luminous stimuli for very short intervals of 

 time. 



Such experiments can be carried out in at least two ways. 

 By a contrivance, similar to a photographic camera shutter, the eye 

 can be stimulated for a very short time, or, the gaze being fixed, a 

 source of light may be moved across the field of vision. For the 

 latter experiment, we may employ a rotating mirror and a pro- 

 jection lantern, or a disc with a slit in it may be rotated in front of 

 a source of light. If the length of the object be I and v be the 

 velocity of movement, then l/v measures the time during which 

 each retinal area is exposed to the action of the stimulus, and by 

 a suitable choice of object and velocity this time becomes as short 

 as we please. 



In principle the methods are alike, but the second is easier in 

 practice, and the results obtained of special interest. 



The earliest observer seems to have been D'Arcy ( 16 ), who, in 

 1765, measured the duration of the response produced by a glow- 

 ing coal which was attached to the circumference of a wheel ; the 

 wheel was rotated faster and faster until a complete circle of light 

 was seen. If a bright object be rotated against a dark back- 

 ground, in the manner described, the whole sensory effect is some- 

 what complicated, comprising under favourable circumstances the 

 following six phases : 



(1) A primary image ; the immediate consequence of the 

 stimulus, its first and strongest effect. As compared with the 

 image due to a stationary illuminant it is more or less elongated 

 into a streak of light. 



(2) Immediately following upon the primary image is a short 

 dark streak. 



(3) After the dark streak we obtain a second phase of illumina- 

 tion which, if the stimulus be coloured, appears complementarily 

 tinged (this is often called, after its discoverer, the Purkinje after- 



