vision. 245 



ing parts of the pictures are thrown on identical points, and 

 referred to identical positions in space. 



Double vision, it may here be mentioned, can be likewise 

 artificially produced when only one eye is used. The prin- 

 ciple, however, is different from that which we have been 

 considering. If, in a card, a few pin holes be made so close 

 together, that they shall be within a space not larger than the 

 aperture of the pupil, and the pin holes be held to the eye, 

 objects at some distance will be seen perfectly; but a minute 

 object, such as a pin head, held near the card will appear mul- 

 tiplied as many times as there are pin holes. The explanation 

 is, that the pin head is out of focus, and, looked at without 

 a diaphragm, would be invisible; but the diaphragm cuts off 

 a large part of each of the pencils of rays which spread out 

 towards the pupil and would have been diffused over an 

 area of the retina so as to interfere with one another; and 

 the perforations admit only very small portions of them, 

 which fall on different parts of that area, and are so narrow 

 that the deficiency in focus is not sufficient to produce con- 

 fusion. 



180. Physiologists have sometimes exercised their ingenuity 

 in trying to account for our seeing objects erect, notwithstand- 

 ing that the pictures on the retina are inverted. But a little 

 reflection will show that the inversion of the retinal image 

 is no reason why the landscape should appear inverted. 

 What we perceive is not the retinal image, but a number of 

 sensations excited by it; and it must be considered as an 

 ultimate fact, that the sensation produced by irritation of a 

 rod or cone of the retina is not perceived as being in that 

 structure, but as situated vertically opposite it, outside the 

 body. If we are to explain why the landscape is not seen 

 inverted, we must explain why it is not seen inside our heads. 

 A child does not rectify an inverted landscape by experience 

 derived from touch, any more than it imagines the external 

 world, as manifested by vision, to be concentrated in two 

 small spots at the bottom of its eyeballs. 



181. Distance, however, is a thing which the eye certainly 

 learns to appreciate by experience. A child, from its entrance 

 into the world, no doubt sees objects as things outside itself; 

 but it learns only by practice the distances of different objects. 



