ELEMENTARY EXPERIMENTAL PHYSIOLOGY 251 



divided up into blue and green in this proportion, revolution will give 

 the hue of the colour complementary to the orange originally selected. 



It will be found by such experiments as this that orange is comple- 

 mentary to greenish-blue, red to bluish-green, yellow to blue, yellowish- 

 green to violet, and green to purple. 



EXPERIMENT II. If a coloured object be viewed on a white surface 

 it may provoke a negative after-image in colour complementary to 

 that of the original object. 



In illustration of this perform the experiments Nos. III. and IV. of 

 Section E in the Milton Bradley Pseudoptics series. 



6. Contrast. Besides the effect which different colours produce 

 when presented simultaneously, or practically simultaneously, to the 

 retina, as in colour-mixing, other effects also will come about when 

 different colours are presented successively and comparatively slowly to 

 the same portion of the retina, or again, when different colours are pre- 

 sented simultaneously to adjacent areas of the retina. 



In the first of these two cases we have the conditions of Successive 

 Contrast, in the second we have Simultaneous Contrast. 



The second experiment in the section on Complementary Colours 

 affords illustration of Successive Contrast. In general the nature of 

 successive contrast may be shown as follows. 



EXPERIMENT I. Take a number of small squares of various colours 

 each about 1 cm. in size. Arrange also a series of fields of different 

 colours, as well as one of white ; these may be squares of 1 or 2 decimetre 

 side. Taking a small red square, place this in the centre of the large 

 white square and in a good light gnze at it for t*o or three minutes. 

 Blow the small object away and continue the gaze. An after-image of 

 the object will be obtained of a colour complementary to that of the 

 original. Substitute for the large white square squares of different 

 colour and perform the experiment again. It will be found that the 

 after-image varies in colour according to the ground on which it is 

 viewed. If red be the colour of the original small square, the after- 

 image on white will be green or bluish-green. If projected on violet 

 the after-image will be blue and if on orange a dull brown. 



EXPERIMENT II. By projection successive contrast may be easily 

 demonstrated as follows. Two lantern slide glass plates are taken, 

 and on one is marked out, in centre of plate, two concentric circles 

 of about 1-5 and 3 cm. radius, enclosed by black lines of just 

 perceptible thickness and having a central dot of about the same 

 2 or 3 mm. diameter. On the second glass plate are fixed rings of 

 coloured gelatine of similar size to the two circular rings, the colours 



