the Sense of Sight. 175 



corona. Only slowly is the state of equilibrium reached ; 

 we apparently have, indeed, a veritable " oscillation " of 

 pressure, alternately within the originally stimulated area and 

 the region immediately surrounding it. 



If, again, the congestion in the gangways between the rods 

 of an illuminated area is not equally distributed throughout 

 their whole length, but is localized, say, at their distal ends, 

 which is, according to the theory, the result when the illu- 

 minating rays are red, the pressure on the surrounding region 

 will be different. It will not affect the whole length of the 

 adjacent rods, but only their distal ends. The pressure 

 exerted on the distal ends of adjacent rods will squeeze the 

 granules which were arranged here both up and down. It is 

 probable that more "will be squeezed up than down, as the 

 downward attraction of the red rays would tend to relieve 

 the pressure at the upper ends of the rods in the illuminated 

 area, and render this the direction of least resistance within 

 the gangways of the adjoining rods. While, then, the red 

 light is massing the granules between the distal ends of the 

 rods in the illuminated area, the pressure caused by these 

 localized assemblages of granules leads to a slight massing of 

 granules above the distal ends of the rods in the surrounding 

 region. Here it should give rise to a different colour-sensa- 

 tion from red. Indeed, it is a necessary corollary of the 

 theory of colour- sensation here proposed that the irradiation 

 fiom any coloured image must be of a different colour. Daily 

 experience shows this to be actually the case. According to 

 this theory, then, the difficult phenomena known as " colour- 

 contrasts " have hardly to be accounted for; they take a 

 natural and necessary place in the scheme *. 



One more point in evidence as to the truth of this theory. 

 It is true that it is again indirect evidence, but its weight 

 cannot be ignored. There has never yet, so far as I know, 

 been any satisfactory explanation of the curious deception 

 presented by what are known as Zollner's parallels. If, 

 however, irradiation is due to a real mechanical pressure, the 

 apparent divergence of these lines admits of very simple 

 explanation. 



We may therefore briefly describe the development of 

 visual organs in the animal kingdom as follows : — 



Under the influence of light certain organisms travelling 

 toward the light seek either to leave the Metazoan body alto- 

 gether or else to discharge their contents at the surface. Such 



* The whiter edge of a white image surrounded by a black border is 

 usually also classed under the head of contrasts ; but this admits of a 

 very different interpretation. 



