io6 2 VISION. 



Irradiation. When a black square on a white ground is compared 

 with a white square of the same size on a black ground, the white square 

 seems the larger. This depends on irradiation, and is seen under various 

 conditions. It is the cause of the apparent notch seen in the edge of a 

 dark object against a flame. Owing to its relatively extensive contour, 

 print is a favourable object for observing the influence of irradiation. 

 The phenomenon has been ascribed to various causes, such as propaga- 

 tion of the physiological process to surrounding elements, spherical 

 aberration, and diffraction at the pupil. There is little doubt that it 

 is a physical phenomenon due to diffusion, for it is much more marked 

 with a large pupil. Printed type appears very considerably reduced 

 in size with a pupil dilated by atropin, and is restored to its normal 

 appearance by means of a small artificial pupil. 1 Irradiation has also 

 been referred to chromatic aberration, but it is said to occur with 

 monochromatic light. 



Homochromatic induction. 2 Under certain circumstances, an in- 

 duced colour may be of the same hue as the inducing colour. This 

 occurs especially with small black or white surfaces on a strongly 

 coloured ground. This homochromatic induction probably depends 

 simply on dispersed light (Blix 3 ). Aars 4 has recently described a form 

 of colour mixture which depends on the same phenomenon. He found 

 that a narrow band of colour seen from a distance is altered in colour- 

 tone when it separates two larger coloured surfaces, the induction being 

 homochromatic. The phenomenon occurred most readily with inducing 

 colours of short wave-length, a fact in favour of dependence on dis- 

 persion'; and the mixture described by Aars is essentially of the same 

 nature as that which occurs on mixing pigments together. 



Temporal and spatial induction. Both after-image and simul- 

 taneous contrast may be regarded as manifestations of a process of 

 physiological induction. Stimulation of any retinal area induces an 

 altered condition of excitability in that area (temporal induction), and an 

 altered condition in surrounding areas (spatial induction). The change 

 in the appearance of a surface during fixation, the after-image, and 

 successive contrast, are examples of temporal induction. Simultaneous 

 contrast, and some other appearances which depend on reciprocal action 

 of different retinal areas, are examples of spatial or areal induction. The 

 corona of an after-image depends on both temporal and spatial induction. 



Bering 5 has used induction in a different and more limited sense. 

 By simultaneous induction he means the change during fixation in 

 adjoining black and white surfaces; by successive induction, the 

 persistence of the change after removal of the stimulating surfaces. 



Theory of after-image. There have been two chief rival theories 

 of after-image. In the theory first advanced by Scherffer, of which 

 Fechner and Helmholtz have been the chief advocates, it is supposed 

 that after-images are due to fatigue ; that the retina is relatively passive ; 

 that in the case of light the after-image is due to general diminished 

 activity of the fatigued area ; while in the case of colour it is due to 



1 Mind, London and Edinb., 1896, vol. , p. 71. 



2 This phenomenon has been named by Blix, " isochromatic induction." Isochromatic 

 is not only a term used in physics in a different sense, but is also incorrect in this 

 connection, meaning rather "of equal colour value." 



3 Skandin. Arch. f. PhysioL, Leipzig, 1895, Bd. v. S. 13. 



4 Ber. internal. Cong. PsychoL, Miinchen, 1896, S. 188. 



5 "Zur Lehre vom Lichtsinn," S. 36. 



