864 TIME-RELATIONS OF RETINAL STIMULATION. 



Not infrequently, after intense stimulation of the retina, positive 

 and negative after-images alternate, until they gradually fuse. Thus, 

 after gazing at the dark-red, setting sun, one sees alternately discs of 

 red and green. In the peripheral portions of the retina, the contrast- 

 phenomena undergo some modification on account of the partial color- 

 blindness that exists in these areas. 



Irradiation is the term applied to certain phenomena that are the 

 result of false estimates of visual sensations due to inexact accommo- 

 dation. If, for example, the edges of objects are thrown upon the 

 retina in diffusion- circles, the mind has a tendency to add the blurred 

 edge to that part of the image which is the most prominent. Bright 

 things appear larger and more prominent than dark ones, and an object 

 itself, without reference to brightness or color, appears more prominent 

 than the background. In the exercise of sharp accommodation the 

 phenomenon of irradiation is not present. 



"A dark object appears smaller than a bright one of the same size. If one 

 look at the same time at a white circle on a black background, and a black circle 

 on a white background, both of the same diameter, at some distance from the eye, 

 the latter will appear to be one-fifth smaller than the former. If the black circle 

 be made that much larger, both will appear of the same size. Tycho de Brahe 

 observed that the moon appeared one-fifth smaller when in conjunction (dark) 

 than when in opposition (full moon). The first quarter of the moon appears to 

 belong to a larger disc than the dark part adjoining it which can often be 

 distinguished at the time of the new moon. Dark clothing makes persons appear 

 much thinner than light clothing. Lights seen behind an edge make an apparent 

 indentation therein. A ruler held before a candle-light seems to be notched. 

 The rising and setting sun appears to make a depression in the horizon" (Goethe}. 



By simultaneous contrast is understood, in the first place, the phe- 

 nomenon that when light and dark parts are present in an image at the 

 same time the light (white) parts always appear the more intense the 

 greater the absence of light from the immediate vicinity, consequently 

 the darker the latter; and conversely the light parts appear the less 

 bright the greater the degree in which white tones are present around 

 them. The analogous phenomenon in connection with colored pictures 

 belongs in the same category : a color in a picture appears the more intense 

 the more completely this color is absent from the immediate neighbor- 

 hood, that is the more the neighborhood contains the tones of the 

 contrast-color. The simultaneous contrast arises thus from two im- 

 pressions simultaneously existing side by side and affecting two different 

 but adjoining portions of the retina. 



Examples of the contrast for light and dark: (i) If a white grating be viewed 

 upon a black background, the points of intersection of the white lines appear 

 darker, because the least amount of black is present in their vicinity. (2) If 

 a point in a narrow strip of dark-gray paper be viewed against a dark background 

 and a large piece of white paper is then inserted between the two, the strip appears 

 much darker than before ; if the white paper be removed, the strip immediately 

 appears brighter. (3) The following is also a most instructive experiment: 

 If, first, a grayish-white surface for example, the ceiling of a room be looked at 

 with both eyes, and then, after a time, a paper tube blackened on the inside, about 

 as long as the hand, and a finger's breadth in diameter, be brought before one eye: 

 the part of the ceiling seen through the tube appears as a round, bright spot. 



Examples of contrast for colors: (i) If a piece of gray paper be placed on a red, 

 yellow or blue background, it appears immediately in the contrast-color, respec- 

 tively green, blue or yellow. The appearance is still more distinct, if the whole 

 is covered quickly with transparent tracing paper. Under similar conditions 

 printed characters on a colored background appear in the complementary color. 

 (2) An air-bubble in the deeply stained field of a thick microscopical preparation 



