POPULAR FALLACIES. 93 



charged more or less with cloudy matter ; the bluish tints are absorbed, and 

 the predominance of the red light transmitted. 



There is perhaps no sense which more requires the vigilant exercise of the 

 understanding to rectify its impressions, than that of sight. The susceptibility 

 of the organ of vision itself is liable to frequent and rapid change, and the 

 same objects at different times produce upon it extremely different' impres- 

 sions. A situation in which, in one condition of the eye, we shall appear to 

 be in absolute darkness, will present to us, in another state of the organ, suffi- 

 cient light to render visible the objects around us. If we are suddenly de- 

 prived of the illumination of any strong artificial light, we appear to be for the 

 moment in absolute darkness ; but when the organ of vision has had time to 

 recover itself, we often find that there is sufficient light to guide us. 



" Thus when the lamp that lighted 



The traveller at first goes out, 

 He feels awhile benighted, 

 And lingers on in fear and doubt. 



' But soon, the prospect clearing, 



In cloudless starlight on he treads, 

 And finds no lamp so cheering 



As that light which heaven sheds." 



THOMAS MOORE. 



The mechanism which the all-wise Artisan that made the eye has 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 of 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 dependant 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 ex- 

 pand, and will presently become so enlarged that enough of light will be re- 

 ceived into the eye to render the objects around us 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 enlargement of the pupil which has taken place to ac- 

 commodate 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. 



The perception we receive of the color of an object depends 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 colors, the sensibility of the retina may be 

 so modified that the same object will appear at different times to have different 

 colors, 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 



