PERCEPTION OE COLOUR. 



?ras contrived to meet these contingencies is marked by the same 

 perfection that prevails through all His works. The opening in 

 the front of the eye, called the pupil, through which light is 

 -admitted to produce vision, is surrounded by an elastic ring, 

 called the iris, which is capable of being contracted or enlarged 

 by the action of certain muscles with which it is connected. It 

 is the magnitude of this opening that determines the quantity ot 

 light transmitted to the retina. If, then, we are in a room 

 illuminated with a strong lamp, the muscles which govern the 

 opening of the pupil contract its dimensions until so much light 

 only is admitted as is consistent with the healthful condition of 

 the eye. If the lamp be suddenly extinguished, and the room 

 be left dependent only on the light admitted by the windows, 

 from the nocturnal firmament, we shall at first appear to be in 

 profound darkness, but immediately the pupil will begin to 

 expand, and -will presently become so enlarged that enough of 

 light will be received into the eye to render the objects around 

 \is faintly visible. 



If in this condition of the organ the lamp again be suddenly 

 brought into the room, the eye will be pained by its light, and 

 the eyelid will immediately drop to give it relief ; for the enlarge- 

 ment of the pupil which has taken place to accommodate it to 

 the faint light to which it was previously exposed, will admit so 

 great a quantity of the strong light of the lamp as to hurt the 

 retina, and the contraction of the pupil cannot be effected with 

 sufficient rapidity to protect the organ from this injury. But 

 the beneficent Maker of the eye has provided for this purpose 

 the eyelid, which is capable of closing instantaneously, and 

 which gives the pupil time to contract, and to accommodate its 

 dimensions to the new condition to which it is exposed. 



5. The perception we receive of the colour of an object 

 \lepends often as much on the condition of the eye when the 

 object is seen as upon the object itself. By the action of lights 

 of different colours, the sensibility of the retina may be so modi- 

 fied that the same object will appear at different times to have 

 .litferent colours, and unreal objects will often be perceived. 

 These are called spectra. If we place on a sheet of white 

 paper a red wafer, and, illuminating, it strongly, direct the eye 

 steadily to it for a short time, and then look at the paper close 

 beside it, we shall there see a blue wafer of the same size. This 

 object is an optical spectrum. The cause of its appearance is 

 easily explained. By the action of the strong red light pro- 

 ceeding from the wafer, the retina is rendered for the moment 

 insensible to the operation of a more feeble red light upon it, for 

 the same reason as the ear would be insensible to the ticking 



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