PHYSIOLOGICAL PARADOXES. 



blue and rose would produce purple, or red 

 and yellow would produce orange. 



Asked, then, what colour they would ex- 

 pect to see if they covered the light of a dark- 

 room lamp first with a yellow sheet of glass and 

 then with a blue one over the yellow, most 

 people would be inclined, if the suspicion of a 

 catch did not hinder them, to say that the 

 light would look green. This, however, it 

 would not do. 



Yellow glass looks yellow for two reasons : 

 because it lets yellow light come through, and 

 because it absorbs and stops all other rays. The 

 second reason is quite as important as the first. 

 Similarly, blue glass is blue, not only because 

 it lets blue rays pass, but because it stops all 

 others. Through the yellow glass, therefore, 

 the blue one receives no blue rays, nor any others 

 except the yellow. Receiving no blue rays 

 it can pass none. The yellow rays which it 

 does receive it cannot pass, because it is in 

 virtue of its inability to pass any rays except 

 blue ones that it is blue. And it receives no 

 others. It has, therefore, no rays that it can 

 pass at all. 



In other words, a light behind both blue 

 and yellow glass should be made black or dark 

 if the two colours are pure and deep and there 

 is no refraction from the surfaces of the glass. 

 And the result is sufficiently near blackness to 

 surprise those who look for green. 



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