THE EYE. 



illuminated, or be not of a sufficiently bright colour to impress the 

 retina sensibly, it will then, instead of appearing as a continuous 

 line of colour, cease to be visible altogether ; for it does not 

 remain in any one position long enough to produce a sensible 

 effect upon the retina. It is for this reason that a ball projected 

 from a cannon or a musket, though passing before the eye, cannot 

 be seen. If two railway trains pass each other with a certain 

 velocity, a person looking out of the window of one of them will 

 be unable to see the other. If the velocity be very moderate, and 

 the light of the day sufficiently strong, the appearance of the 

 passing train will be that of a flash of colour formed by the 

 mixture of the prevailing colours of the vehicles composing it. 



An expedient has been contrived, depending on this principle, 

 to show experimentally that the mixture of the seven prismatic 

 colours, in their proper proportions, produces white light. The 

 colours are laid upon a circular disk surrounding its edge, which 

 they divide into parts proportional to the spaces they occupy in 

 the spectrum. When the disk is made to revolve, each colour 

 produces, like the lighted stick, the impression of a continuous 

 ring, and consequently the eye is sensible of seven rings of the 

 several colours superposed one upon the other, which thus produce 

 the effect of their combination, and appear as white or a whitish 

 grey colour. 



64. The duration of the impression upon the retina, after the 

 object producing it is removed, varies according to the vividness 

 of the light proceeding from the object, being longer according as 

 the light is more intense. It was found that the light proceeding 

 from a piece of coal in combustion moved in a circle at a distance 

 of 165 feet, produced the impression of a continuous circle of 

 light when it revolved at the rate of seven times per second. The 

 inference from this would be that in that particular case the 

 impression upon the retina was continued during the seventh part 

 of a second after the removal of the object. 



It is from the cause here indicated that forked lightning pre- 

 sents the appearance of a continuous line of light. 



65. The duration of the impression on the retina varies also 

 with the colour of the light, that produced by a white object 

 being most visible, and yellow and red being most in degree of 

 durability ; the least durable being those tints which belong to 

 the most refrangible lights. 



66. The duration of the impression also depends on the state of 

 illumination of the surrounding space ; thus the impression pro- 

 duced by a luminous object when in a dark room is more durable 

 than that which would be produced by the same object seen in an 

 illuminated room. This may be ascribed to the greater sensitiveness 



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