COLOUR VISION. 269 



shall be able to explain this. The yellow solution cuts off the blue end of 

 the spectrum, leaving only the red, orange, yellow, and green. The blue solution 

 cuts off the red end, leaving only the green, blue, and violet. The only light 

 which can get through both is the green light, as you see. In the same way 

 most blue and yellow paints, when mixed, appear green. The light which falls 

 on the mixture is so beaten about between the yellow particles and the blue 

 that the only light which survives is the green. But yellow and blue light 

 when mixed do not make green, as you will see if we allow them to fall on 

 the same part of the screen together. 



It is a striking illustration of our mental processes that many persons have 

 not only gone on believing, on the evidence of the mixture of pigments, that 

 blue and yellow make green, but that they have even persuaded themselves 

 that they could detect the separate sensations of blueness and of yellowness 

 in the sensation of green. 



We have availed ourselves hitherto of the analysis of light by coloured 

 substances. We must now return, still under the guidance of Newton, to the 

 prismatic spectrum. Newton not only 



" Untwisted all the shining robe of day," 



but shewed how to put it together again. We have here a pure spectrum, 

 but instead of catching it on a screen we allow it to pass through a lens 

 large enough to receive all the coloured rays. These rays proceed, according 

 to well-known principles in optics, to form an image of the prism on a screen 

 placed at the proper distance. This image is formed by rays of all colours, 

 and you see the result is white. But if I stop any of the coloured rays the 

 image is no longer white, but coloured ; and if I only let through rays of 

 one colour, the image of the prism appears of that colour. 



I have here an arrangement of slits by which I can select one, two, or 

 three portions of the light of the spectrum, and allow them to form an image 

 of the prism while all the rest are stopped. This gives me a perfect command 

 of the colours of the spectrum, and I can produce on the screen every possible 

 shade of colour by adjusting the breadth and the position of the slits through 

 which the light passes. I can also, by interposing a lens in the passage of 

 the light, shew you a magnified image of the slits, by which you will see the 

 different kinds of light which compose the mixture. 



The colours are at present red, green, and blue, and the mixture of the 



