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A TEXTBOOK OF PHYSIOLOGY 



a quick and correct answer was immediately given to the question 

 as to which colour was uppermost. 



A white object seen on a dark background appears larger than a 

 black object of equal size on a white background. This is known as 



irradiation (Fig. 359). When a white 

 strip is placed between two black 

 strips, the edges of the white strip near 

 the black appear whiter than the 

 middle portion; the centre of a white 

 cross placed on a black background 

 appears shaded. A white dog may 

 appear clean indoors, but very dirty 

 when out on newly fallen snow. The 

 phenomenon is known as simultaneous 

 contrast. This contrast is also expe- 

 rienced with colours. A neutral grey 

 strip surrounded by green seems to 



acquire a pinkish hue, the tint of the complementary colour to green 

 (red). If surrounded by blue the acquired hue of the neutral grey 

 object is yellow. If a grey strip surrounded by green be viewed 

 intently for some time and the gaze then transferred to a sheet of 

 white paper of other white surface, the after-image of the strip will 

 then appear greenish surrounded by red. This is known as successive 

 contrast. Various speculations are put forward to account for the 

 phenomena. According to the Helinholtz view of colour -vision such 

 contrast effects are cerebral in origin, and in reality are errors of 



FIG. 359. To DEMONSTRATE 

 IRRADIATION. 



The white spaces appear larger than 

 the black. They are the same 

 size. 



B. L. R. 



FIG. 360. RIGHT- AND LEFT-EYED IMAGES OF TRUNCATED PYRAMID. 



, Truncated cone; L, S, right- and left-handed images. By fixing the vision beyond 

 the book, L and P may be made to combine and give B. 



judgment. According to the Hering view they are retinal in origin 

 due to modified metabolic effects of the visual substances. 



If a solid object be viewed fixedly by either eye separately, and 

 then by both eyes, it is easily appreciated that separately the eyes view 

 the object from a different standpoint. Together they view the 

 object from the combined standpoint, and we have in addition an 

 increased sensation of solidity and perspective. This is well seen by 

 looking vertically downwards at the truncated pyramid. With both 

 eyes it has the appearance of B, with the left only it is seen as L, and 

 with the right eye only as E (Fig. 360). 



