POPULAR FALLACIES. 



optical spectrum. The cause of its appearance is easily explained. By the ac- 

 tion of the strong red light proceeding 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 of a clock imme- 

 diately after being affected by a discharge of 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 are not per- 

 ceived, 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 ob- 

 jects will appear orange. 



The eye may be, and sometimes is, either from disease, or from original im- 

 perfection of structure, either imperfectly sensible or altogether insensible to 

 lights of particular colors. To such eyes all objects will appear to have col- 

 ors different from those which they present to organs of vision in the usual 

 healthy state. We can thus easily understand the condition of a jaundiced eye. 

 Such an organ is more or less insensible to the blue and red lights, but highly 

 sensitive to the yellow. White objects to such an eye will appear yellow, and 

 all other objects will appear in tints different from their proper colors, and par- 

 taking more or less the yellow hue. 



Instances have more than once occurred, and are recorded in the works on 

 optics, of individuals thus incapable, from original defects of vision, of perceiv- 

 ing particular colors. The late Doctor Dalton, of Manchester, was a conspic- 

 uous example of this. 



But, as we have above stated, even a healthy and perfect eye will be ren- 

 dered temporarily insensible to the impression of particular colors by being 

 exposed for a short time to the strong action of colored lights. Optical illu- 

 sions 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 toward 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 colors, 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 dne 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 eyelids then be 

 closed, a blue spectrum of the sun will be seen, and will continue to be visible 

 until the retina recover its state of repose. 



If we write a page or two with red ink, and then commence to write with 

 black ink, the writing will appear of a light blue color, and will continue to ap- 

 pear so until the retina loses the impression made by the red ink upon it. In 



