782 EXAMPLES OF CONTRAST. 



2. An air-bubble in the strongly tinged field of vision of a thick microscopical preparation 

 appears with an intense contrast colour {Landois). 



3 Paste four green sectors upon a rotatory white disc, leave a ring round the centre of the 

 disc' uncovered by green, and cover it with a black strip. On rotating such a disc the black 

 part appears red "and not grey {Brilckc). . . . ; ' ' 



4 Look with both eyes towards a greyish -white surface, and place in front of one eye a tube 

 about the length and breadth of a finger, composed of transparent oiled paper, gummed together 

 to such thickuess as will permit light to pass through its walls. The part of the surface seen 

 through the tube appears in its contrast colour. The experiment also shows the contrast in the 

 intensity of the illumination {Landois). A white piece of paper, with a round black spot in its 

 centre, when looked at through a blue glass appears blue with a black spot. If a white spot of 

 the same size on a black ground be placed in front, so that it is reflected in the glass plate and 

 just covers the black s|x)t, it shows the contrast colour yellow (Jiagona Scina). 



5. The coloured shadows also belong to the group of simultaneous contrasts. " Two condi- 

 tions are necessary for the production of coloured shadows firstly, that the light gives some 

 kind of a colour to the white surface ; second, that the shadow is illuminated, to a certain 

 extent, by another light. During the twilight, place a short lighted candle on a white surface, 

 between ft and the fading daylight hold a pencil vertically, so that the shadow thrown by the 

 candle is illuminated, but not abolished, by the feeble daylight ; the shadow appears of a 

 beautiful blue. The blue shadow is easily seen, but it requires a little attention to observe that 

 the white paper acts like a reddish-yellow surface, whereby the blue colour apparent to the eye 

 is improved. One of the most beautiful cases of coloured shadows is seen in connection with 

 the full moon. The light of the candle and that of the moon can be completely equalised. 

 Both shadows can be obtained of equal strength and distinctness, so that both colours are 

 completely balanced. Place the plate opposite the light of the moon, the lighted candle a little 

 to one side at a suitable distance. In front of the plate hold an opaque body, when a double 

 shadow apiears, the one thrown by the moon and lighted by the candle being bright reddish- 

 yellow ; and, conversely, the one thrown by the candle and lighted by the moon appears of a 

 beautiful blue. Where the two shadows come together and unite is black" {Goetlie). 



6. ' ' Take a plate of green glass of considerable thickness and hold it so as to get the bars of 

 a window reflected in it, the bars will be seen double, the image formed by the under surface 

 of the glass being green, while the image coming from the under surface of the glass, and which 

 ought really to be colourless, appears to be purple. The experiment may be performed with a 

 vessel filled with water, with a mirror at its base. With pure water colourless images are 

 obtained, while by colouring the water coloured images are produced" {Goethe). 



Explanation of Contrast. Some of these phenomena may be explained as due to an error of 

 judgment During the simultaneous action of several impressions, the judgment errs, so that 

 when an effect occurs at one place, this acts to the slightest extent in the neighbouring parts. 

 When, therefore, brightness acts upon a part of the retina, the judgment ascribes the smallest 

 possible action of the brightness to the adjoining parts of the retina. It is the same with 

 colours. It is far more probable that the phenomena are to be referred to actual physiological 

 processes {Hering). Partial stimulation with light affects not only the part so acted on, but also 

 tlic surrounding area of the retina (p. 782); the part directly excited undergoing increased dis- 

 assimilation, the (indirectly stimulated) adjoining area undergoing increased assimilation; 

 the increase of the latter is greatest in the immediate neighbourhood of the illuminated 

 portion, and rapidly diminishes as the distance from it increases. By the increase of the 

 assimilation in those parts not acted on by the image of the object, this is prevented, so that 

 the diffused light is perceived. The increase of the assimilation in the immediate neighbourhood 

 of the illuminated spot is greatest, so that the perception of this relatively stronger different 

 light is largely rendered impossible {Hering). 



[Helmholtz thus ascribed the phenomena of contrast to psychical conditions, i.e., errors of 

 judgment, but this explanation is certainly not complete. A far more satisfactory solution of the 

 problem is that of Hering, that stimulation of one part of the retina affects the condition of 

 adjoining parts. If a white disc on a black background be looked at for a time, and then the 

 eyes be closed, a negative after-image of the disc appears, but it is darker and blacker than the 

 visual area, and it has a light area around, brightest close to the disc, i.e., the adjacent part of 

 the retina is affected. This Hering has called successive light induction.] 



Successive Contrast Look for a long time at a dark or bright object, or at a coloured {e.g., 

 red) one, and then allow the effect of the contrast to occur on the retina, i.e., with reference 

 to the above, bright and dark, or the contrast colour green, then these become very intense. 

 This phenomenon has also been called "successive contrast." In this case the negative after- 

 image obviously plays a part. 



[Some drugs cause subjective visual sensations, but these do so by acting on the brain, e.g., 

 alcohol, as in delirium tremens, cannabis indica, sodic salicylate, and large doses of digitalis 

 {Brunton).] 6 



399. MOVEMENTS OF THE EYEBALLS EYE MUSCLES. The globular 



