355. =Near-Sighted and Far-Sighted.=--Some persons have their eyes so shaped that they can not fully adjust them to objects at different distances. Thus the near-sighted can see with distinctness only objects that are near. The reason is that the rays converge too much, and are brought to a focus before they arrive at the retina, as represented in Fig. 247. The images therefore of distant objects are indistinct. If the retina could in any way be brought forward a little the difficulty would be obviated. But as this can not be done, concave glasses are resorted to, which counteract the effect of the too highly refractive power of the eye. In the far-sighted the difficulty is of an opposite character. The refractive power is so feeble that when near objects are viewed the rays are not brought to a focus soon enough, as seen in Fig. 248. Convex glasses are used in this case, making the divergent rays of near objects less divergent before they enter the cornea. 356. =Images in the Eye Inverted.=--The images formed on the retina are inverted. This can be proved by taking the eye of an ox and carefully paring off the back of it, leaving little else than the retina itself. Holding now a candle before the eye, its image may be seen inverted upon its rear part. The question arises why it is that we see objects erect when their images on the retina are inverted. On this point I will quote from my _Human Physiology_: "It has been supposed by some that we really see every thing reversed, and that our experience with the sense of touch, in connection with that of vision, sets us right in this particular. And this it is supposed is the more readily done from the fact that our own limbs and bodies are reversed as pictured on the retina, as well as objects that are around us, so that every thing is _relatively_ right in position. But if this be the true explanation, those who have their sight restored after having been blind from birth should at first see every thing wrong side up, and should be conscious of rectifying the error by looking at their own limbs and bodies. But this is not so. The above explanation of erect vision, and other explanations of a similar character, are based upon a wrong idea of the office which the nerve performs in the process of vision. It is not the image formed upon the retina which is transmitted to the brain, but an impression produced by that image. The mind does not look in upon the eye and see the image, but it receives an impression from it through the nerve; and this impression is so managed that the mind gets the right idea of the relative position of objects. Of the way in which this is done we know as little as we know of the nature of the impression itself."