380 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



because these are " symmetrical " with the pores of the eye aloue. 

 This correspondence is the basis of sense specificity. Further 

 there is a symmetrical arrangement within the eye itself with 

 respect to the different forms of stimulation. By means of the 

 intra-ocular fire we perceive the emanations of fire i.e. white 

 with the " water " we see water i.e. black and so on. 



" With earth we see earth, with water we see water, with air 

 we see the bright air ; just as with love we (perceive) love, and 

 with hate, baleful hate " ( 1 ). Empedocles is said to have regarded 

 four colours, white, black, red, and green, as primaries (Stobaeus), 

 but only examines white and black in detail. He also taught 

 that rays issued from the visual " fire," but how this process was 

 associated with his general doctrine of pores and emanations is 

 not certain. 



Democritus (? 460-357) agreed with Empedocles in postulating 

 the entrance of particles from an object into pores contained in the 

 perceiving structure and in the dictum that " like is perceived by 

 like." But he denied that there are four qualitatively distinct 

 elements, and believed that all things are made up of homogeneous 

 atoms moving in a vacuum and infinitely numerous. Vision is 

 due to the mirroring of an object in the eye, the latter's character 

 being somehow determined by its moist and porous nature. This 

 part of Democritus' theory was sharply criticised by Aristotle, 

 who remarked : " It is absurd also that it should not have occurred 

 to him to doubt why the eye alone sees, but nothing else in which 

 energies are apparent. That the sight is aqueous is true ; yet it 

 does not happen that it sees because it is aqueous but because it 

 is diaphanous, which is also common to air " ( 2 ). 



Democritus seems to have been the first writer to attempt a 

 detailed theory of colours, the simple ones being white, black, red, 

 and green ; his account, which is somewhat elaborate, has not 

 played a sufficiently important part in the history of opinion to 

 need further description, but it is well to remember that he (anti- 

 cipating Berkeley l ) held that colour had no objective reality. 

 "... the ultimate elements the plenum and the vacuum are 

 destitute of all sensible qualities, while the things composed of 

 them possess colour (as they do every sensible quality) owing 

 merely to the order, figure, and position of the atoms, i.e. (a) to their 



1 Cf. "The First Dialogue between Hylas and Philonous," especially pp. 314- 

 318 (Sampson's edition, vol. i.). Lucretius, loc. cit. 



