

 328 INDEPENDENT ACTION OF EACH HEMISPHERE. 



but in vision with both eyes the faults of the more imperfect one are 

 merged in the indications of the better, and the same might be remark- 

 ed of the ear ; from which it would appear that this doubleness of or- 

 gans is rather for the purpose of introducing a principle of compensation 

 than one of conspiring action, the object intended to be gained being a 

 justness of perception rather than an increase of effect. 



These observations apply to double organs in their normal states, or, 

 Effect of tem- if not their normal, their habitual ones ; but if to the eye, 

 porary disturb- f QY exam pl e a temporary disturbance is given, as by press- 



anceofoneor- f . , . , ,. , n 



gan. ure which renders its optical axis oblique, the fellow organ 



being permitted to retain its usual position, double sight is the result. 

 It is true that, in the habitual divergence of strabismus, such is not the 

 effect, one of the images disappearing, or perhaps the mind, accommodat- 

 ing itself to the habitual condition, combines the two into one. These 

 circumstances indicate that each member of a double organ can, under 

 conditions of disturbance, exercise an independent and even opposing ac- 

 tion to its fellow. 



It has by some been supposed that the mind pays attention to the im- 

 The indications pressions of only one of the pair of organs at a time ; thus, 

 contem fated *^at we see ^ ie ^ ma g es furnished by only one eye, though we 

 at a time. can with very great quickness direct attention to those fur- 

 nished by the other, and therefore, deceived by the rapidity with which 

 this alternation of attention can be accomplished, our belief in the syn- 

 chronous use of both organs is an error. If two differently colored ob- 

 jects, such as differently tinted wafers, be so placed as to be separately 

 and yet simultaneously viewed by both eyes, the mind vainly attempts 

 to combine the two images together. We do not see the resulting form 

 of a green tint, but we see, according as our attention is given to the 

 right or left, a blue or a yellow, if these have been the colors of the wa- 

 fers, and these colors can quickly merge into one another, like dissolving 

 Illustrative views. There is a simple experiment which serves to support 

 experiment. t m ' s view, and which any one may readily make. If the open 

 hand be placed along the nose, so as to divide the right eye from the left, 

 and we look upon the surface of a uniformly-illuminated sheet of paper 

 covered with writing, it will be found that we can only read with one 

 eye at a time, but that the mind can with great rapidity determine which 

 eye it will use. In this little experiment, we have, moreover, the means 

 of estimating the relative sensitiveness of the two eyes, and other of their 

 optical peculiarities ; thus it will be commonly remarked that, though the 

 paper be, as we have said, uniformly illuminated, that part of it which is 

 regarded by one eye is brighter than that seen by the other, this being 

 due to a difference in their sensibility. It will also frequently occur 

 that the two portions of the page will present different shades of tint, 



