POPULAR FALLACIES. 



of a clock immediately after being affected by a discharge ot 

 artillery. Accordingly, when the eye, after viewing the red 

 wafer, looks at a white paper beside it, the action of that portion 

 of the compound white light reflected from the paper which is 

 red fails to produce any perception, and the remaining constituents 

 alone are perceived, which accordingly present a bluish tint. 

 To comprehend this, and other similar illusions, it is very 

 necessary to remember that white light is a compound of reds, 

 yellows, and blues, and that if we deprive it of any one of 

 these elements it will assume the tint produced by the others. 

 Thus, if the eye be insensible to red light, all white objects 

 will appear to it with a tint composed of yellow and blue. If 

 it be insensible to blue light, then white objects will appear 

 orange. 



Instances have more than once occurred, and are recorded 

 in the works on optics, of individuals incapable, from ori- 

 ginal defects of vision, of perceiving particular colours. The 

 late Dr. Dalton, of Manchester, was a conspicuous example of 

 this. 



But, as we have above stated, even a healthy and perfect eye 

 will be rendered temporarily insensible to the impression ot 

 particular colours by being exposed for a short time to the strong 

 action of coloured lights. Optical illusions are produced in this 

 way in the exhibition of fireworks. When luminous balls, some 

 red and some white, are thrown up into the air, the white appear 

 blue beside the red, and are generally imagined to be really blue. 

 The effect, however, is a visual illusion, ascribable to the cause 

 just explained. 



In the sky towards sunset, when reddish clouds are arranged 

 with openings between them, the sky at such openings appears 

 green, although it be really blue. 



In astronomical observations on the stars there is a curious 

 case, in which it has never been settled whether the appearance 

 is real or illusive. Many of the stars, which to the eye appear 

 individual objects, prove to be double when examined with 

 powerful telescopes. The two stars, thus composing a double 

 star, are frequently of different colours, and it is found that 

 when one is red the other is of a bluish tint. Now we know that 

 it would appear of this tint, even though it were a white object, 

 by reason of the presence of the red star. Whether, in these 

 cases of double stars, the blue one would be really blue, or is 

 rendered so by the optical effect adverted to, has not been 

 decided, it being impossible to view it except in juxtaposition 

 with its red companion. 



If the eye be directed to the sun for a few seconds, and the 

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