This light travels in straight lines and, striking objects before us, is reflected in all directions.  Some of these rays passing through a point situated behind the lenses of the eye, strike the retina.  The multiplication of these rays on the retina produces a picture of whatever is before the eye, such as can be seen on the ground glass at the back of a photographer’s camera, or on the table of a camera obscura, both of which instruments are constructed roughly on the same principle as the human eye.

These rays of light when reflected from an object, and again when passing through the atmosphere, undergo certain modifications.  Should the object be a red one, the yellow, green, and blue rays, all, in fact, except the red rays, are absorbed by the object, while the red is allowed to escape.  These red rays striking the retina produce certain effects which convey to our consciousness the sensation of red, and we say “That is a red object.”  But there may be particles of moisture or dust in the air that will modify the red rays so that by the time they reach the eye they may be somewhat different.  This modification is naturally most effective when a large amount of atmosphere has to be passed through, and in things very distant the colour of the natural object is often entirely lost, to be replaced by atmospheric colours, as we see in distant mountains when the air is not perfectly clear.  But we must not stray into the fascinating province of colour.

What chiefly concerns us here is the fact that the pictures on our retinas are flat, of two dimensions, the same as the canvas on which we paint.  If you examine these visual pictures without any prejudice, as one may with a camera obscura, you will see that they are composed of masses of colour in infinite variety and complexity, of different shapes and gradations, and with many varieties of edges; giving to the eye the illusion of nature with actual depths and distances, although one knows all the time that it is a flat table on which one is looking.

Seeing then that our eyes have only flat pictures containing two-dimension information about the objective world, from whence is this knowledge of distance and the solidity of things?  How do we see the third dimension, the depth and thickness, by means of flat pictures of two dimensions?

The power to judge distance is due principally to our possessing two eyes situated in slightly different positions, from which we get two views of objects, and also to the power possessed by the eyes of focussing at different distances, others being out of focus for the time being.  In a picture the eyes can only focus at one distance (the distance the eye is from the plane of the picture when you are looking at it), and this is one of the chief causes of the perennial difficulty in painting backgrounds.  In nature they are out of focus when one is looking at an object, but in a painting the background is necessarily on the same focal plane as the object.  Numerous are the devices resorted to by painters to overcome this difficulty, but they do not concern us here.