fact, that if we put our heads between our legs, or lie down on a hill head downwards, we nevertheless see objects in their right position, and not upside down; although the portion of the retina which is usually met by the lower part of the object is then met by the upper: in fact, everything is topsy turvy excepting the Understanding.
The second thing which the Understanding does in converting sensation into perception, is to make a single per ception out of a double sensation ; for each eye in fact receives its own separate impression from the object we are looking at ; each even in a slightly different direction : nevertheless that object presents itself as a single one. This can only take place in the Understanding, and the process by which it is brought about is the following : Our eyes are never quite parallel, excepting when we look at a distant object, i.e. one which is more than 200 feet from us. At other times they are both directed towards the object we are viewing, whereby they converge, so as to make the lines proceeding from each eye to the exact point of the object on which it is fixed, form an angle, called the optic angle; the lines themselves are called optic axes. Now, when the object lies straight before us, these lines exactly impinge upon the centre of each retina, therefore in two points which correspond exactly to each other in each eye. The Understanding, whose only business it is to look for the cause of all things, at once recognises the impression as coming from a single outside point, although here the sensation is double, and attributes it to one cause, which therefore presents itself as a single object. For all that is perceived by us, is perceived as a cause—that is to say, as the cause of an effect we have experienced, consequently in the Understanding. As, nevertheless, we take in not only a single point, but a consider able surface of the object with both eyes, and yet perceive it as a single object, it will be necessary to pursue this
