Halleck says regarding this "sight without seeing" idea: "A body may be imaged on the retina without insuring perception. There must be an effort to concentrate the attention upon the many things which the world presents to our senses. A man once said to the pupils of a large school, all of whom had seen cows: 'I should like to find out how many of you know whether a cow's ears are above, below, behind, or in front of her horns. I want only those pupils to raise their hands who are sure about the position and who will promise to give a dollar to charity if they answer wrong.' Only two hands were raised. Their owners had drawn cows and in order to do that had been forced to concentrate their attention upon the animals. Fifteen pupils were sure that they had seen cats climb trees and descend them. There was unanimity of opinion that the cats went up heads first. When asked whether the cats came down head or tail first, the majority were sure that the cats descended as they were never known to do. Any one who had ever noticed the shape of the claws of any beast of prey could have answered the question without seeing an actual descent. Farmers' boys who have often seen cows and horses lie down and rise, are seldom sure whether the animals rise with their fore or hind feet first, or whether the habit of the horse agrees with that of the cow in this respect. The elm tree has about its leaf a peculiarity which all ought to notice the first time they see it, and yet only about five per cent of a certain school could incorporate in a drawing this peculiarity, although it is so easily outlined on paper. Perception, to achieve satisfactory results, must summon the will to its aid to concentrate the attention. Only the smallest part of what falls upon our senses at any time is actually perceived."