SIGHT. 577 



Plateau's well known optical illusion the phantascope, Prof. 

 Wheatstone's kaleidophone, and the revolving mirror by which 

 he has measured the velocity of electricity and the duration of 

 the electric spark, are all applications of this physiological law. 



Ocular spectra. When the eye has been fatigued by looking 

 at any particular kind of coloured light, and is afterwards directed 

 to a white surface, the surface will not appear white, but of a 

 different colour, which is called the accidental colour of the colour 

 which was first regarded by the eye. This fact may be proved 

 by placing a red wafer on a sheet of white paper, and fixing the 

 eye for some time steadily to a dot at its centre : when the eye is 

 turned aside to an uncovered part of the paper, a circular spot of 

 the same size as the wafer will be seen, but its colour will be 

 green. This image changes its position as the eye moves, and is 

 called an ocular spectrum. 



To determine the accidental colour of any colour originally 

 presented to the eye, the following remarks must be attended to. 

 There are three primary and distinct colours, red, yellow, and 

 blue ; and all the compound colours that exist in nature are dif- 

 ferent combinations of these. Orange is a compound of red and 

 yellow; green, of yellow and blue ; purple, of blue and red ; and 

 white is a neutral combination of the three primary colours. An 

 accidental colour is always found to be that which, added to the 

 original colour, produces white ; and these two colours are hence 

 said to be complementary. 



The theories which have been advanced to account for the 

 phenomena of ocular spectra may be reduced to two. 



The most usual theory in its most general form supposes that, 

 when any simple or compound colour is continuously presented 

 to the eye, the part of the retina where the image falls becomes 

 less sensible to impressions of the same kind, but retains its sus- 

 ceptibility for other kinds of impressions : if, therefore, while the 

 retina is in this state, the eye is transferred to a white surface, 

 the spectrum will appear as if the colour originally seen were 

 subtracted from the white. This explanation agrees with a great 

 many observed facts, but there is one which seems in direct con- 

 tradiction to it. A complementary spectrum is seen when the 

 eye is shut, and when, consequently, there is no white light to 

 furnish the complementary colour. This phenomenon has given 

 rise to another explanation as to the origin of ocular spectra, 



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