If conversation turns to judging a picture which is finally dismissed as worthless, one often hears at the end of the clever talk, "Well, anyway, I couldn't do it. I can't even draw a straight line."
The speaker, humbly sympathetic and respectful of the painter's ability, is usually speaking from a quite unfounded sense of his own incompetence. Even if he does not mean a literally "straight" line, the layman often credits the practiced painter with a mass of secret gifts, particularly the mysterious gift of "talent" for drawing.
This awe is quite unjustified. The average person at school and at work is assumed to have much greater ability to perform in several directions than what is needed simply to draw what he sees. Of course, some guidance is needed to acquire this ability, just as it is for writing, building, or cooking. A professional painter or draftsman naturally needs education and practice, just as does any craftsman, doctor, or administrator. Only very outstanding accomplishment is a matter of talent.
The famous straight line cannot be drawn without a ruler even by the greatest artist. If you test the lines in the drawings of great masters with a ruler you will see at once that no line is perfectly straight, that lines that seem parallel are not always, and that verticals are very rarely truly vertical. This is the case even with pictures of buildings which were themselves made with plumb line and level, and seem to be so in the free, unconstrained drawing, in spite of its demonstrably crooked and wavy lines.
In this connection let us see where truly straight lines are to be found in nature. There is, in fact, only one example: the sun's rays when they are made visible by the moisture in the atmosphere. Another example, the line of the horizon on the sea, does not generally appear straight because $hadows and reflections are often concentrated near the horizon. They cause an optical curve, up or down, of the line.
One of the first rules of all picture making is to draw not what you know, but what you see at the moment. This is equally true of external sight and of interior vision, the "inner eye."
The straight rays of the sun, the only straight lines in nature, are often drawn with a ruler, which makes a delightful and natural contrast to an otherwise free representation. Apart from this, truly straight lines occur only in man-made structures, in architecture. Sometimes in a painting, buildings also are sketched in with the aid of ruler and protractor. But many artists are enraged at the mere mention of the ruler. In this matter you must not be influenced by the expression of other peoples' feelings, but do what you feel is right for you in your own work.
Straight lines also help in the construction of spatial perspective, but the artist will seldom use a ruler to make them; in any case he will eliminate them once they have served as controls, or scaffolding, for his structure.
Generally, a free drawing of a building by,a practiced draftsman looks more natural and competent than the constructional drawing of an architect. No one can say with scientific accuracy why this should be so, since the mysteries of the human organism of sight are involved here. The way the human eye combines the impressions it receives is understood to some degree, but the significance the brain attaches to these combined impressions is a matter of psychology.
Nothing differs so much from one individual to another as the eye's judgment, the ability to evaluate and judge measurements, angles, horizontals, and verticals. People do not react equally to the influence of optical illusion. These peculiarities of the process of sight, though not universally identical, have, like all other human senses, a constant tendency to equalize themselves, to work towards an average.
This explains why, in the classic experiment we have illustrated, the heavier lines are seen to be true horizontals and parallel to each other only when one screws up one's eyes until the weaker sloping lines cannot be clearly' seen. You may have noticed how painters often screw up their eyes while they are working. They do this in order to shut out confusing, peripheral impressions and isolate the essential effect, of color as much as of form.