7 8q after-images. 



" That the impression of any picture remains for some time upon the eye is a physio- 

 lotncal phenomenon \ when such an impression can be seen for a long .time, it becomes patho- 

 logical. The weaker the eye is, the longer the image remains upon it. The retina does not 

 recover itself so quickly, and we may regard the action as a kind of paralysis. This is not to 

 be wondered at in the case of dazzling pictures. After looking at the sun the image may 

 remain on the retina for several days. A similar result sometimes occurs with pictures which 

 are not dazzling. Buseh records that the impression of an engraving, with all its details, 

 remained on his eve for 17 minutes " {GoctJu). , 



Experiments and Apparatus for Positive After-images. 1. When a burning stick is rapidly 

 rotated, it appears as a fiery circle. 



2 The phanakistoscope (Plateau) or the stroboscopic discs (Stampfcr). Upon a disc or 

 cylinder, a series of objects is so depicted that successive drawings represent individual factors 

 of one continuous movement. On looking through an opening at such a disc rotated rapidly, 

 we see pictures of the different phases moving so quickly that each rapidly follows the one in 

 front of it. As the impression of the one picture remains until the following one takes its 

 place, it has the appearance as if the successive phases of the movement were continuous, and 

 one and the same figure. The apparatus under the name of zoetrope, which is extensively used 

 as a toy, is generally stated to have been invented in 1832. It was described by Cardanus in 

 1550. * It may be used to represent certain movements, e.g., of the spermatozoa and ciliary 

 motion, the movements of the heart and those of locomotion. 



3. The colour top contains on the sectors of its disc the colours which are to be mixed. As 

 the colour of each sector leaves a condition of excitation for the whole duration of a revolution, 

 :ill the colours must be perceived simultaneously, i.e., as a mixed colour. 



[Illusions of Motion. Silvanus P. Thompson points out that if a series of concentric circles 

 in black and white be made on paper, and the sheet on which the circles are drawn be moved 

 with a motion, as if one were rinsing out a pail, but with a very minute radius, then all the 

 circles appear to rotate with the same angular velocity as that imparted. Professor Thompson 

 has contrived other forms of this illusion, in the form of strobic discs.] 



Negative After-images. Occasionally, when the stimulation of the retina is 

 strong and very intense, a " negative," instead of a positive after-image, appears. In 

 a negative after-image, the bright parts of the object appear dark, and the coloured 

 parts in corresponding contrast colours (p. 774). 



Examples of Negative After-images. After looking for a long time at a dazzlingly-illuininated 

 white window, on closing the eyes we have the impression of a bright cross, or crosses, as the 

 case may be, with dark panes. 



Negative coloured after-images are beautifully shown by Norrenberg's apparatus. Look 

 steadily at a coloured surface, e.g., a yellow board with a small blue square attached to the 

 centre of its surface. A white screen is allowed to fall suddenly in front of the board the white 

 surface now has a bluish appearance, with a yellow square in its centre. 



The usual explanation of dark negative after-images is that the retinal elements are fatigued 

 by the light, so that for some time they become less excitable, and consequently light is but 

 feebly perceived in the corresponding areas of the retina ; hence, darkness prevails. 



Hering explains the dark after-images as due to a process of assimilation in the black-white 

 visual substance. In explaining coloured after-images, the Young-Helmholtz theory assumes 

 that, under the action of the light waves, e.g., red, the retinal elements connected with the 

 perception of this colour are paralysed. On now looking suddenly on a white surface, the 

 mixture of all the colours appears as white minus red, i.e., the white appears green. In bright 

 daylight the contrast colour lies very near the complementary colour. According to Hering, 

 the contrast after-image is explained by the assimilation of the corresponding coloured visual 

 substance, in this case, of the "red-green" ( 397). From the commencement of a momentary 

 illumination until the appearance of an after-image, 0'344 sec. elapses (v. Vintschgau and 

 Lustig). 



Not unfrequently, after intense stimulation of the retina, positive and negative 

 after-images alternate with each other until they gradually fuse. After looking at 

 the dark-red setting sun we see alternate discs of red and green. 



The phenomena of contrast undergo some modification in the peripheral areas of 

 the retina, owing to the partial colour-blindness which occurs in these areas 

 (Adamiick and Woinow). 



Irradiation is the term applied to certain phenomena where we form a false 

 estimate of visual impressions, owing to inexact accommodation. If, from inexact 

 accommodation, the margins of the object are projected upon the retina in diffusion 

 circles, the mind tends to add the undefined margin to those parts of the visual 



