164 STRUGGLE OF THE TWO FIELDS. [BOOK in. 



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 struggle between the two 

 colours, now one, and now the other, becoming prominent, inter- 

 mediate tints however being frequently passed through. This 

 may arise from the difficulty of accommodating at the same time 

 for the two different colours ( 735) ; both eyes will be accom- 

 modated 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. 



