Vision^ tracing them to Functional Actions of the Biain. 253 



From the latter part of this, and also fz'om the second ex- 

 periment, an important piece of knowledge is derived ; namely, 

 that "whatever he the state of the sensibility in the one eye, that 

 state is exactly reversed at the same time i?i the other eye. Hence, 

 if the deep green appearance of S' in this last experiment is 

 owing to an excess of the perception of the green light, the 

 bright whitish appearance of S'' must be derived from a de- 

 fective perception of the green light. We know that the green 

 colour of the slip of paper used in this experiment was not 

 owing to an absolute defect of the red and other primary 

 coloured rays, but to an excess or predominance of the oreen 

 light reflected from it. The whitish appearance of S", there- 

 fore, might be the result either of insensibility to this excess 

 of green, or of an increase of sensibility to the light which is 

 complementary to green. The whitish appearance, therefore, 

 observed by the shaded eye in the fifth experiment is alto- 

 gether equivocal, unless strict attention is paid to its degree^ 

 as will be jnanifest from tlie following considerations. White 

 is the result of a mixture of any two antagonist or comple- 

 mentary colours in certain jiroportions. Let us, for the sake 

 of illustration, suppose these proportions to be equal ; then 

 one part of red light and one part of complementary green, or 

 two parts of each, or, in short, any equal parts of each, will 

 form 'white, the brilliancy of which will be as the number of 

 rays composing it. Now suppose a surface which appears 

 green to contain one part oi red and five of g?een, it is obvious 

 tliat if this surface is converted to white by an addition of red 

 light, it will be five times brighter than if it is made white by 

 a removal of gree?i light. The white colour of the green slip 

 to the shaded eye, in the fifth experiment, appeared to me 

 much brighter than could be accounted for by insensibility to 

 the superabundant green light. But to remove all uncertainty 

 on the subject, I had recourse to the following experiment. 



Exp. 6. I laid on a deep black ground a slip, about one 

 twentieth part of an inch in breadth, of the same green paper 

 which 1 had used in the last experiment,, in a straight line 

 with a slip of white paper of the same breadth. I viewed them 

 both through a piisni held parallel to their length, and ob- 

 serving thai the red margin appeared much broader and 

 brighter in the white than in the green slip, I shaded the 

 white one with China ink, by degrees, until the red border in 

 it appeared exactly like that of the green slip, expecting to 

 obtain in this manner a very correct sj)ecimen of the deo^ree 

 of whiteness which the green slip used in the last experiment 

 would assume, if the eye was insensible to the predominant 

 green rays : that it was so, there cannot, on reflectioii, be any 

 doubt. I therefore used this shaded slip as a test on repeating 



