Drawing

Drawing of Leonardo Da Vinci
It is necessary to say something about Vision in the first place, if we are to have any grasp of the idea of form.
An act of vision is not so simple a matter as the student who asked her master if she should “paint nature as she saw nature” would seem to have thought. And his answer, “Yes, madam, provided you don’t see nature as you paint nature,” expressed the first difficulty the student of painting has to face: the difficulty of learning to see.
Let us roughly examine what we know of vision. Science tells us that all objects are made visible to us by means of light; and that white light, by which we see things in what may be called their normal aspect, is composed of all the colours of the solar spectrum, as may be seen in a rainbow; a phenomenon caused, as everybody knows, by the sun’s rays being split up into their component parts.
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.
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