THE SENSES 103 



more rapid vibrations act specially upon the nerve endings 

 in the eye to produce molecular changes which in turn affect 

 the centres in the brain and give rise to changes in con- 

 sciousness which we call light. The range of vibrations 

 which can act in this way is comparatively limited, the 

 slowest being about 435 billions per second, the most rapid 

 about 764 billions. Vibrations more rapid than this, which 

 are capable of setting up chemical changes, as in photo- 

 graphy, do not act upon the eye. 



The action of light upon the protoplasm of lower organ- 

 isms has been already considered (p. 16), and it has been 

 seen that it may be either general or unilateral, producing 

 the phenomenon of positive or negative phototaxis. In more 

 complex animals special sets of cells are specially set aside to 

 be acted on by light, and these are generally imbedded in 

 pigmented cells to prevent the passage of light through the 

 protoplasm. Such an accumulation of cells constitutes an 

 eye, and in the simpler organisms such an eye can have no 

 further function than to enable the presence or absence of light 

 or various degrees of illumination to produce their effects. 



But in the higher animals these cells are so arranged that 

 certain of them are stimulated by light coming in one direc- 

 tion, others are stimulated by light coming in another, and 

 while the former are connected with one set of synapses in 

 the brain, the latter are connected with another. Thus light 

 coming from one point will stimulate one set of cells which 

 will excite one part of the brain, and light from another will 

 act upon other cells which will excite another part of the 

 brain, and thus not merely the degree of illumination but the 

 source of illumination becomes distinguishable. 



It is by this arrangement that it becomes possible to form 

 ideas of the shape of external objects. One directs the eye 

 to the corner of the ceiling, and the idea that it is a corner 

 is due to the fact that three different degrees of illumination 

 are appreciated, and that these can be localised one above, 

 one to the right, and one to the left. One set of cells is 

 stimulated to one degree, another set of cells to another 

 degree, and a third set of cells to a third degree ; and the 

 different stimulation of these different sets of cells leads to a 

 different excitation of separate sets of cells in the brain. 



