PRACTICAL PHYSIOLOGY. [LXXIV. 



two images are in strong contrast, while the two surrounding areas scarcely 

 contrast at all. (Bering's apparatus is made by Rothe, Wenzelbad, Prague.) 



There are two theories of contrast, viz., that of Helmholtz, the 

 "psychological theory," and the "physiological theory," of which 

 Hering is the chief supporter. Hering has devised many experiments 

 in support of his contention. The former theory represents contrast 

 as due to an error of judgment. On the physiological theory, Hering 

 supposes that there are material chemical changes in a hypothetical 

 retino-cerebral "vision-stuff" (" Seh-stoff "), These changes may 

 be assimilative (anabolic) (black, blue, green), or dissimilative 

 (katabolic) (white, yellow, red). A change in one area may 

 influence the retino-cerebral apparatus outside the area directly 

 affected by the stimulus. 



7. Hering's Experiment on Simultaneous Contrast. 



Divide a large quadrilateral sheet of paper vertically into halves, 

 and make one half black and the other white. Near the centre of 

 the vertical division gum two V-shaped pieces of grey paper (one 

 on the black and the other on the white half) with their apices 

 together. The V on the black looks lighter by contrast than that 

 on the white. Fix the V's for a minute, and then look at a uniform 

 surface. Even after the after-image of the back grounds has 

 disappeared, the after-image of the V on the black ground looks 

 darker than that of the V on the light ground. This, Hering con- 

 tends, must be due to a material change taking place in a localised 

 part of the retino-cerebral apparatus. It seems difficult to explain 

 this result as dependent upon an error of judgment due to the 

 influence of the background. Hering regards this as a fundamental 

 experiment in support of his theory. Similar experiments may be 

 made with coloured papers. 



8. Successive Light Induction ( Hering}. 



(a. ) Look for one minute at a small white circular disc on a black back- 

 ground, e.g., velvet. Close and cover the eyes. A negative after-image of 

 the disc appears, but it is darker and blacker than the visual area, and it has 

 a peculiar light area round it, brightest close to the disc, and fading away 

 from it. 



(b.) Look at two small white square patches of paper placed one-eighth of 

 an inch apart on a black background. On closing the eyes, the black space 

 between them looks brighter than the other three sides of the squares. 



(c.) Look at a black strip on a white ground. On closing the eyes there is 

 no partial darkening of the white ground, but only an intensely bright image 

 of the strip. 



9. Coloured Shadows. 



(a.) Place an opaque vertical rod (i inch in diam. ) in front of a white back- 

 ground. Admit not too bright daylight to cast a shadow of the rod. Place 

 a lighted candle behind one side of the rod, the shadow caused by the yellow- 



