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intently for a few seconds on a brightly illuminated coloured 

 object is well known, the colour complementary to the one 

 looked at always appears when the gaze is removed to a 

 colourless surface. It is also a matter of common observa- 

 tion that when the eyes have been directed to a bright 

 light for a short time, the image left on the retina as seen 

 when the eyes are averted is dark; but if the eyes are 

 rapidly opened and closed the image is still seen bright. 

 I am not aware, however, that it has ever been noticed that 

 this image varies in size according to the distance of the 

 background to which the eyes are directed. A circle of 

 gas jets, perhaps, affords the simplest test. It will be seen 

 after looking at the circle of light for a few seconds — (in 

 some cases a more or less lengthened gaze at the light is 

 necessary, owing to the varying sensitiveness of the retina) 

 - that, if the vision be turned to a distant background, the 

 size of the image is instantly enlarged, and then, if the eyes 

 be directed to a near background, the image is reduced in 

 size. If any difficulty should be found in seeing the reversed 

 image of the gas jets, it may readily be seen as a bright 

 object by rapidly closing and opening the eyelids. The 

 effect is the same as if the image were seen through a cone 

 — the apex of the cone being held close to the eyes. In 

 other words, the effect is the reverse of the ordinary rules 

 of perspective. 



