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than in a particular manner, but that they can be produced in that 

 particular manner, and that when so produced, their union affects 

 the eye with no sensation of greenness. 



Let two very narrow strips of white paper, A, B, be placed parallel 

 to one another in sunshine, so as to be seen projected on a perfectly 

 black ground (a hollow shadow), and viewed through a prism having 

 the refracting edge parallel to them, the refraction being towards the 

 eye, and let the nearer B be gradually removed towards A, so that 

 the red portions of B's spectrum shall fall upon the green portion of 

 A's. Their union will produce yellow, or, if too far advanced, 

 orange. On the other hand, it will be seen that the yellow space in 

 B's spectrum on which the blue of A's falls is replaced by a streak 

 of white, whiteness, and not greenness, being the resultant of the 

 joint action of these rays on the retina. If the strips be made 

 wedge-shaped, tapering to fine points, and A being still white, B be 

 made of paper coloured with the yellow chromate of mercury before 

 mentioned, the whiteness of the streak where the blue of A mixes 

 with the yellow of B near the pointed extremities will be very 

 striking. 



f There is a certain shade of cobalt-blue glass which insulates, or 

 very nearly so, a definite yellow ray from the rest of the spectrum, 

 suppressing the orange and a great deal of the green. If the spec- 

 trum of B, formed and coloured as last described, be viewed through 

 this glass, a very well-defined image of it, clearly separated from its 

 strong red and very faint blue images, will be seen. As the glass in 

 question allows blue rays to pass, the white object, A, besides its 

 definite yellow image, will form a broad blue, indigo, and violet train 

 nearer to the eye. Now let B be gradually brought up towards A, 

 so that the violet, indigo, and blue rays of this train shall coincide in 

 succession with the yellow image of B, no sensation of greenness will 

 arise at any part of its movement. Again, if a white card be laid 

 down on a black surface, the edge nearest the eye, when refracted 

 towards the spectator by a prism, will of course be fringed with the 

 more refrangible half of the spectrum. Let this be viewed through 

 such a glass, and in the blue space so seen introduce one half of a 

 narrow rectangular slip of paper thus coloured, having its upper edge 

 in contact with the lower edge of the white card, the other half pro- 

 jecting laterally beyond the card. In this arrangement the definite 



