THE RETINA. 303 



Thus the eyelids are perpetually and unconsciously closing 

 arid opening with a rapid movement, popularly called " wink- 

 ing. " This movement is for the purpose of cleansing the eye- 

 ball, and, were it not for the image- retaining power of the 

 retina, we should pass a considerable part of our time in abso- 

 lute darkness. As it is, the impression of external objects on 

 the retina lasts longer than the time occupied in winking, and, 

 in consequence, we are not conscious that any interval of dark- 

 ness has elapsed. 



Again, when we have been looking steadfastly at an object, 

 and then move our eyes, the image of that object is seen in the 

 new focus; and it is worthy of notice that such object is 

 always seen in its " complementary " colour. For example, if 

 we have been looking at a scarlet spot, and suddenly move our 

 eyes, we shall see a spot exactly similar in size and shape, but 

 of green. 



I well remember that when I was a boy I was reading with 

 almost feverish anxiety the green handbill of a travelling 

 circus, to which I hoped that I might be allowed to attend. 

 Having finished it, I asked for some note-paper, for the pur- 

 pose of putting my request in writing, but, to my astonishment, 

 mixed, perhaps, with a little irritation, all the paper supplied 

 to me was of a bright pink. For a time no arguments could 

 convince me that the paper was really white, until by degrees 

 the pink hue became paler and paler, and the paper assumed its 

 normal whiteness. 



The fact was, that the eye had become saturated with the 

 green — i.e. the blue and yellow rays — and could see nothing 

 but their complementary colour, which was pink. 



A good example of this property may be found in a lighted 

 stick, which, if rapidly whirled round, appears to form a con- 

 tinuous circle of fire. The reason of this is, that the impression 

 made on the retina by the fiery point does not cease until the 

 stick has again come round in its course. 



Then there are those well-known chromatic tops, in which 

 are inserted pieces of bent wire. When the top is spun 

 these pieces of wire assume exactly the appearance of trans- 

 parent jugs, vases, glasses, and similar articles. A very pretty 

 illustration of this principle is given by a little machine, which 

 is made to revolve rapidly by means of a multiplying wheel. 



