968 STRUGGLE OF THE TWO FIELDS. [Book hi. 



so as to be united into a single perception, the result is not 

 always a mixture of the two impressions, that is a grey, but, in 

 many cases, a sensation similar to that produced when a polished 

 surface, such as plumbago, is looked at: the surface appears 

 brilliant, is said to have a "lustre." The reason probably is 

 because when we look at a polished surface the amount of 

 reflected light which falls upon the retina is generally different 

 in the two eyes ; and hence we associate an unequal stimulation 

 of the two retinas with the idea of a polished lustrous surface. 

 We may in this connection refer to what is known as "the 

 struggle of the two fields of vision," though the matter is one of 

 sensations and not of judgments or intricate psychical processes. 

 When the impressions of two colours are united in binocular 

 vision, the result is in most cases not a mixture of the two 

 colours, as when the same two impressions are brought to bear 

 together at the same time on a single retina, but a struo-o4e 

 between the two colours, now one, and now the other, becoming 

 prominent, intermediate tints however being frequently passed 

 through. This may arise from the difficulty of accommodating 

 at the same time for the two different colours (§ 548) ; both 

 eyes will be accommodated at the same time and to the same 

 degree, but if two eyes, one of which is looking at red, and the 

 other at blue, be at one moment both accommodated for red rays, 

 the red sensation will overpower the blue, while if at another 

 moment they are both accommodated for blue, the blue will 

 prevail. It may be however that the tendency to rhythmic 

 action, so manifest in activity of other simpler forms of living 

 matter makes its appearance also in the cerebral changes involved 

 in binocular vision. 



