SENSE OP SIGHT. 209 



sible spherical, these lines of visible direction meet and cross 

 at a point within the eye, which is called the centre of visible 

 direction ; the lines from the upper part of the image go to 

 the lower part of the object, and those from the lower part 

 of the image proceed to the upper part of the object. Thus 

 an inverted image necessarily produces an erect object, and 

 the external object is the thing to which the mind attends, 

 and not the object on the retina. The error of all these 

 philosophers has consisted in this, that they have imagined 

 a true picture to be formed on the retina, which is regarded 

 by the mind, and therefore seen inverted. But there is no 

 interior eye to see or take cognizance of this image ; but 

 the mind accurately refers the impression made on the 

 retina to the object producing it. In short, the mind stands 

 behind the retina and looks at objects as they are through 

 this screen, but it does not see the screen itself, but judges 

 of the position of objects by the direction in which the light 

 comes from them towards the eye ; and as Arnott well 

 remarks, "no more deems an object to be placed low be. 

 cause its image may be low in the eye, than a man in a 

 room into which a sun-beam enters by a hole in the window- 

 shutter, deems the sun low because its image is on the 

 floor." 



29. Again, why, as we have two eyes, does not every 

 object appear double ? fThe reason is, that in the two eyes 

 are corresponding points, so that when a similar impres- 

 sion is made on both, the sensation, or vision, is single. 

 Now, we have the faculty of so directing the axis of the 

 eyes, that the image of* an object falls exactly on the same 

 point in each ; but if from any cause there is the least dis- 

 turbance, then vision becomes double, as often happens to 

 the drunkard. This is easily known by slightly pressing 

 a finger on the ball of either eye, so as to prevent its following 

 the motion of the other. Persons who squint always have 

 double vision, but then they acquire the power of attending 

 to the sensation in one eye at a time. Animals that have 

 18* 



