896-97-] SPATIAL THRESHOLDS OF COLOURS. 237 



on a black ground, should counterpoise one another in regard to the 

 angular magnitudes at which they are distinguishable definitely in their 

 proper tones. There is no prima facie ground for supposing that an 

 orange and a blue of equal intensity as light and subjected to exactly 

 the same environing conditions of colourless light, or absence of light, 

 should display mutually inverse facility in rendering themselves visible 

 as orange and blue respectively. Hence, that they apparently do so pair 

 off in complementary doubles in regard to the characteristic space 

 threshold, is a noticeable regularity whose explication is not contained 

 analytically in the fact of their being complementary colours. 



Our results on a green ground show a very prevalent tendency for 

 the characteristic thresholds to be lowest in the red and yellow with 

 a lowering somewhat in \.h& purple. On the other hand, the regions of 

 highest characteristic thresholds are the orange^ orange-yellow^ 

 yellow-green and blue-green. We would naturally expect that 

 the contrast influence of the green ground would dispose purple to 

 have the lowest characteristic colour threshold of all the colours because 

 of the fact that from the most minute surface magnitudes of the colour 

 examined, it is continuously inducing thereupon a more or less influen- 

 tial disposition to purple. However, we find that red is the lowest in 

 characteristic threshold, though, in case of one observer, we must state 

 the purple was seen quite as low as the red. 



But if we consider our results on blue and red grounds respectively we 

 find a similar phenomenon with much more declared and precise 

 features in contradiction to what we might be disposed to expect. 



On the red ground we find an unequivocal unanimous tendency 

 among all the observations for the lowest characteristic threshold to be 

 at the blue, and for the colours on both sides in the spectrum to become 

 higher in their characteristic thresholds. The highest mark is reached 

 mainly in the orange, yellow, green and violet, while in the orange- 

 yellow there is a disposition to a lowering though in no case does it 

 reach the low mark of the blue. The blue-green is for the most part 

 low also. Now it must at once strike us as peculiar that on a red 

 ground the blue should be the colour of the lowest spatial threshold. 

 We would naturally expect the blue-green or some of the greens in that 

 neighbourhood to be most easily seen as such and such, and hence to 

 have the smaller threshold because it is the so-called complementary of 

 the red. Being complementary, and therefore having the co-operation 

 of the contrast influence of the red ground to emphasize its peculiar 

 quality of colour we would think it should have a certain primacy over 



