WHAT DO ANIMALS SEE? I55 



from those affecting simple tests such as matching strips of 

 coloured paper. 



A very striking experiment has recently been devised to 

 illustrate this : two black-and-white photographs, not 

 coloured ones, are taken of, say, a garden full of flowers. 

 One photograph is taken through a red filter and the other 

 through a green one. Two exactly similar black-and-white 

 lantern slides are made from the negatives and each is placed 

 in a separate lantern for projection on to the same screen. 

 The lanterns are adjusted so that the two pictures fit each 

 other exactly and form one. A red filter is placed over the 

 lens of the lantern containing the slide made with a red 

 filter; if its light is switched on alone a red picture appears 

 on the screen. The lantern containing the slide made with a 

 green filter has no filter placed over its lens; when it is 

 switched on alone a black-and-white picture appears on the 

 screen. But if you switch on both lanterns together so that 

 the red and the black-and-white pictures fall superimposed 

 on the screen, the picture appears in full colours — the red 

 flowers are red, the leaves and grass are green, the yellow 

 flowers are yellow, the blue ones blue and the white ones 

 white. This astonishing result certainly takes a lot of explain- 

 ing ! Although it may confirm some parts of the old theories 

 of colour vision it seems probable that they will have to be 

 revised or perhaps superseded by a new one. One thing 

 seems certain : that the appearance of colours depends not 

 only on the surrounding field, but on the way the message 

 sent from the cones of the retina is coded in the nerve cells 

 and sent to the brain. It depends, moreover, on the way the 

 message is interpreted by the brain when it is received there. 

 Colour, like beauty, lies not only in the eye of the beholder, 

 but also in the brain : it is subjective as well as objective. 



