their Relation to certain other Visual Phenomena. 



283 



violet and violet having no sensible effect, while an admixture of red, 

 orange, or yellow causes the border to become red. The intensity of 

 the red borders is much greater than that of the blue-green, and if 

 the two could occur together, the blue-green would no doubt be over- 

 powered. According to Hering's theory the red and blue-green 

 fundamental sensations, being antagonistic, cannot both be excited at 

 the same time, and it is to be remarked that those spectral rays which 

 are less refrangible than the greenish-yellow produce red borders, 

 while those of refrangibility intermediate between greenish-yellow and 

 blue-violet produce blue-green borders, which is nearly what the 

 Hering theory would require. According to the most recent exponents 

 of the Young-Helmholtz theory, green spectral rays excite the funda- 

 mental red sensation to about the same extent as orange-red rays ; 

 yet no red border is formed by the green, though that formed by the 

 orange-red is very strong. If the presence of these borders may be 

 taken as affording evidence of the excitation of fundamental colour- 

 sensations, the evidence so far is in favour of Hering's views. But on 

 the other hand the fact that the red borders can be caused by all kinds 

 of white light seems to show that white excites the fundamental red 

 sensation, while there is some evidence in Sections IV and VI that it 

 excites green and blue or violet colour-sensations as well. No indica- 

 tion as to what one or more colour-sensations in addition to red and 

 blue-green are fundamental ones has yet been afforded by the class of 

 border phenomena under discussion. 



Simultaneous Contrast. — When a purple pulsative image of a very 

 bright green patch is formed upon a white ground by the eyepiece 

 method, the whole physically white field appears to be strongly 

 purple, a fact which shows conclusively that the phenomenon of 

 simultaneous contrast may in certain cases be absolutely independent 

 of mental judgment. It cannot be that the ground appears purple 

 simply from contrast with green, for no green whatever is consciously 

 perceived ; the cause must necessarily be a physiological one. Similar 

 remarks apply to the orange and yellow fields which accompany the 

 pulsative images of blue and violet patches. It is curious that with 

 a red patch the colour of the field is but very slightly affected. 



But while these observations show that in certain cases the so- 

 called contrast effects must have a physiological origin, it is beyond 

 question that this is not invariably so. Some of Helmholtz's well-known 

 experiments leave no room for doubt that mental judgment is some- 

 times the sole cause of contrast phenomena. 



Colours of the Pulsative Image. — The chief results of the colour 

 experiments are collected in Table II. One of the most noticeable 

 features is the superior intensity of the pulsative after-images of red 

 and green; another is the intrusiveness of some form of purple. 

 Purple after green is, as before mentioned, more easily obtainable than 



VOL. LXVIII. X 



