STIMULATION THE SENSES 117 



that it must always be going on, even while light is acting. Hence 

 there is a balance between the decomposing action of light and the 

 recombination of the products, such that the composition of the 

 system depends on the intensity of the illumination. Since this 

 balance only lasts as long as external energy of light is being 

 supplied, it is not a true chemical equilibrium. The reason why 

 the image formed on a photographic plate does not disappear after 

 exposure is because there is gelatin present, and the chlorine or 

 bromine liberated combines with the gelatin, and is not available to 

 recombine with the silver. 



Other phenomena, whose meaning is not yet clear, are produced 

 in the retina by light (P., pp. 519-525). Among these there is an 

 interesting electrical change. 



The perception of colour is a question about which opinions are 

 somewhat at variance. When we look at the spectrum there are 

 to most of us six distinct colours in it red, orange, yellow, green, 

 blue, and violet. A few people, like Newton, see a distinct colour, 

 indigo, between blue and violet. All other colours can be formed 

 by combinations of these with each other and with black or white. 

 Whether there is a distinct variety of visual purple for each of the 

 six primary colours, or whether each of these affects the same 

 substance in a different way, is unknown. It is certain, however, 

 that visual purple, as we know it, absorbs light of all parts of the 

 spectrum ; but this may be due to its being a mixture of six 

 substances. 



Position-receptors 



These receptors are of two kinds, and they give us information 

 of our position in relation to the direction of gravity or of the 

 direction in which our bodies have moved. They may be called 

 position-receptors, and make use either of the weight of particles 

 to stimulate nerve endings, or of the inertia of liquid in its refusal 

 to take up suddenly the movement of a vessel which contains it. 



Practically all multicellular animals, and plants too, have organs 

 by which the direction of gravity is made known to them, and 

 reactions set up to bring them into a definite position in relation to 

 it. We find sacs full of liquid containing one or more solid 

 particles. Projecting into the liquid are hairs attached to cells in 

 connection with nerve fibres. Such organs are known as statocysts 

 in animals (E., p. 214). In plants, starch grains in cells appear to 

 perform similar functions. When the organ or cell is in the normal 

 position in relation to the vertical, the grains lie on the lowest part 

 of the sacs and stimulate the nerves or protoplasm in that region. 

 If the position changes, the grains stimulate hairs in a different place, 

 and the change of position is known and corrected. This mode of 



