The angle formed between any objects and our eye is the only means by which our sight estimates their size and distance, and as this angle is the simple effect of complex causes, the judgment we form does not distinguish between the several causes; we are compelled to be inaccurate. For how can I tell, by sight alone, whether the angle at which an object appears to me smaller than another, indicates that it is really smaller or that it is further off. Here we must just reverse our former plan. Instead of simplifying the sensation, always reinforce it and verify it by means of another sense. Subject the eye to the hand, and, so to speak, restrain the precipitation of the former sense by the slower and more reasoned pace of the latter. For want of this sort of practice our sight measurements are very imperfect. We cannot correctly, and at a glance, estimate height, length, breadth, and distance; and the fact that engineers, surveyors, architects, masons, and painters are generally quicker to see and better able to estimate distances correctly, proves that the fault is not in our eyes, but in our use of them. Their occupations give them the training we lack, and they check the equivocal results of the angle of vision by its accompanying experiences, which determine the relations of the two causes of this angle for their eyes. Children will always do anything that keeps them moving freely. There are countless ways of rousing their interest in measuring, perceiving, and estimating distance. There is a very tall cherry tree; how shall we gather the cherries? Will the ladder in the barn be big enough? There is a wide stream; how shall we get to the other side? Would one of the wooden planks in the yard reach from bank to bank? From our windows we want to fish in the moat; how many yards of line are required? I want to make a swing between two trees; will two fathoms of cord be enough? They tell me our room in the new house will be twenty-five feet square; do you think it will be big enough for us? Will it be larger than this? We are very hungry; here are two villages, which can we get to first for our dinner?