Page:Optics.djvu/131

 145. We pass over the question which has embarrassed many: "why external objects appear erect to the eye, whereas their images, by which it is supposed we judge of them, are inverted." People have debated this point very earnestly, and reasoned on it at great length, appearing to consider these images as something real that we could see or feel: the fact is merely this, that in vision the rays of light are collected to different points on the retina, and that by the various sensations there produced by them, we are informed of the existence of objects without us, probably in a manner analogous to that in which we are made sensible of those or other objects, by sensations excited in the organs of hearing.

We judge of the relative places of visible objects by the relative places of their images in the bottom of the eye, and it is probable that experience teaches us to connect corresponding phænomena in this as in many other cases, though it is not mentioned, we believe, in any account of persons having their sight suddenly restored, that they were at all at a loss as to the position of objects at first.

Some writers have endeavoured to explain why the two images, formed by our two eyes, do not excite in us the idea of two objects instead of one. We can only conjecture that the sensations excited in corresponding parts of the retinas are melted as it were into one, where the two optic nerves unite. Perhaps it is merely experience that leads us to form a correct judgment. Cheselden in his Anatomy gives an account of a person who had one of his eyes distorted by a blow, so that every object seemed double to him for some time, but by degrees he recovered his single vision, first of familiar objects, and afterwards of all others, though the distortion always remained. Now in this case, the images could not be formed on corresponding parts of the retinas, and moreover, the same sensation seems, at different times, to give rise to double and to single vision, the only difference being due to habit.

Persons who squint do not direct both eyes to the same object, yet their vision is single, and what is more remarkable, this defect is sometimes acquired and sometimes cured, without double vision being experienced.

146. While we are on the subject of sensations in the retina we may observe that there is one spot in the eye which is insensible,