SENSE OF SIGHT. 869 



taneously with the two eyes, the question arises, Why is it that with 

 two retinae, each capable of receiving a distinct image, the image so 

 formed by the use of the two eyes is not double ? The explanation of 

 this lies not so much in the anatomical distribution of the tunics of 

 the eye as in the fact that when rays of light proceed from a luminous 

 body and fall upon the retinas they fall upon parts that are accustomed 

 to act together. Every point of the image on one retina falls upon a 

 coincident point upon the other. These points are accustomed to act 

 together and we see a single image. The eyes, indeed, receive a double 

 impression, and this may be illustrated by placing two small bodies, 

 as the fingers, at different distances from the eyes. Directing the eyes 

 to the nearer, the more remote will appear double, or, if the eye be 

 directed to the more remote, the nearer will appear double. This is 

 due to the fact that the images of the object upon which the eye is not 

 especially directed do not fall upon coincident parts of the retina. 

 The same thing may be accomplished by throwing the two parts of the 

 retina out of coincidence with each other, when, almost instantaneously, 

 double vision will occur. If, while looking at a single object, we press 

 to one side the globe of one eye, we bring two parts of the retina to act 

 together which did not ordinarily thus coincide, and double vision ensues. 

 Any other cause, such as various poisons, disease, or fatigue, which will 

 disturb the co-ordination of the eyes or such an adjustment of the posi- 

 tion of the eyes that rays of light do not fall upon corresponding or 

 coincident parts, will result in double vision. 



Again, it has been stated' that the image formed upon the retina is 

 an inverted one, and yet it is a matter of common experience that every 

 body is seen in the field of vision as upright. This second mental rever- 

 sion of the image is the result of experience acquired in the exercise of 

 •the sense of sight. It is not to be understood that the brain takes cog- 

 nizance of the picture upon the retina and that vision is an actual trans- 

 mission of this image to the sensorium. The mind does not look upon 

 the picture so formed, but vision is a mental act excited by an impres- 

 sion upon the optic nerve whose neurility is excited. There need be no 

 ■more correspondence between the change in the brain and the image in 

 the eyes than between the signs of the telegraph operator and the words 

 of the written message. It must be remembered that vision consists in 

 the change developed in the central organ as a consequence of changes 

 in the optic nerve. Even supposing that the image on the retina is 

 inverted, what difference does it make? Everything is inverted and the 

 relative position of things is unchanged. All objects hold the same rela- 

 tion to each other whether the image be inverted or erect ; hence the 

 mind is not conscious of any inversion. 



Although each retina receives an image from the object the pictures 



