ELEMENTARY EXPERIMENTAL PHYSIOLOGY 253 



The above experiments on Complementary Colour and Contrast 

 depend upon variations in excitability in the retinal area involved or in 

 adjacent retinal areas. The change in excitability that occurs in any 

 retinal area when affected by incident light is spoken of as caused by 

 temporal induction, and the change that is brought about in adjacent 

 areas as resulting from spatial induction. Successive contrast depends 

 upon temporal induction, simultaneous contrast upon spatial induction. 

 The phenomena connected with the formation of after-images are 

 examples mainly of temporal induction. 



With regard to the complementary colour of after-images, this 

 is thought by some to be simply the result of fatigue. Others regard 

 the phenomena as due to initiation of processes, the converse of those 

 brought about by the original stimulus. Bering's theory of colour 

 vision involves an explanation of these processes in accordance with the 

 latter view. 



In this connection it will not be out of place to refer to a pheno- 

 menon known as Irradiation. 



EXPERIMENT V. Let a black square be inscribed in a white square 

 of three times the size, and conversely, let a white square be 

 inscribed in a black square of three times the size. The side of the 

 inner square will be equal and should be about a centimetre long If 

 the two figures be placed side by side, the inner white square will 

 appear larger than the inner black square. The material for this 

 experiment on a larger scale is also provided in the Milton Bradley 

 Pseudoptics, Section C, Experiment IV. The explanation of this may be 

 due to the dispersive power of the lens, as the appearance is more con- 

 spicuous with a large pupil, or it may be due to the chemical processes 

 of a certain kind (katabolic) in the retina tending to encroach on 

 adjacent fields of the retina, the opposite processes (anabolic) apparently 

 not having that tendency. 



EXPERIMENT VI. A line passing through the adjacent edges of tv^o 

 rows of black squares, arranged so as to overlap appears oblique. See 

 Milton Bradley Pseudoptics, Section B, Experiment V. 



7. Colour Blindness. The inability to distinguish different hues of 

 colours constitutes the condition of colour blindness. It may vary 

 much as regards the failure shown. A person may be red blind and then 

 only appreciates the colour of red objects as far as they show other con- 

 stituents of white light. Such a person, according to the Helmholtz 

 theory of colour vision, would be entirely lacking in the production of 

 the red sensation. Or a person may lack the green sensation and be 

 green blind, and very rarely violet blindness may exist. 



