vii KETINAL EXCITATION 359 



Persistence of luminous sensations after the stimulus has 

 ceased occurs also when the retina is excited by lights of different 

 colours. According to Charpentier, each colour acts in proportion 

 to its luminous intensity. The positive after-image of a mono- 

 chromatic light shows the colour of the stimulating light, which 

 gradually fades till it disappears into the background. 



Negative after-images, which result from fatigue of the elements 

 of the retina, must be distinguished from positive after-images. 



When a light object on a dark ground is fixated for a certain 

 time (5-15 sees.), and the eye is then directed steadily to a 

 larger surface which is uniformly bright and moderately illumin- 

 ated, a part of this, which corresponds approximately to the 

 form of the object fixated and to the portion of the retina excited, 

 and therefore fatigued, appears dark in relation to the rest. This 

 dark area, which reproduces the outlines and details of the object 

 fixated the more plainly in proportion to the difference between 

 its light and dark parts, is the negative image of it. It depends 

 on the fact that the fatigued points of the retina are less excitable 

 than the points which are not fatigued, or resting. 



These negative after-images are common in daily life, but as 

 they are incomplete, and our eyes are in constant motion, they 

 generally escape notice. The duration of the negative images 

 increases with the period of fixation and the luminous intensity 

 of the object, and with the difference in illumination between the 

 object and the background. 



It is to be noted that after-images are relatively positive or< 

 negative, not absolutely so : the after-image of a very bright 

 object is at the same time positive (bright) on closing the eyes, 

 and negative (dark) on gazing at a well-illuminated surface. 



The after-image of a bright object changes in absolute dark- 

 ness from positive to negative, and afterwards a series of light 

 and dark images alternate, until, after 4-5 minutes, the continuous 

 sensation of a faint grey light alone remains. This is known as 

 Plateau's oscillations of after-images, which had been previously 

 described by Purkinje. They result from automatic variations of 

 retinal activity, due to the periodical oscillations of its excitability, 

 which are independent of the action of light. 



VI. The positive and negative changes in visual sensibility, 

 induced by different degrees of light or darkness, which Aubert 

 termed retinal adaptation, are intimately connected with the 

 phenomena of fatigue and recovery of the sensitive elements of the 

 retina. 



It is a matter of common observation that in passing rapidly 

 from a good light into semi-darkness, one sees very badly at first, 

 as though the darkness were total ; after a time objects become 

 indistinctly visible, and finally clearer, until after a few moments 

 we are able to distinguish most details. This is known as adapta- 



2 A 2 



