Suppose, that we went away from the tent, camp or home, the width and length of which are known to us, and we want to check, how far have we gone. Or we come back from a trip and, having seen a house or a tent from a distance, we want to know, how much way is still ahead of us. When we do not have any equipment with us, we only use our hand and eye. We extend our right hand in front of us and, closing our left eye, we fix the end of the index finger on the left side of the house; next, trying to keep my hand completely still, we close our right eye quickly, a otwieramy life; then the tip of the finger will seemingly move to the right side and stand, e.g.. on the half of the frontage of the house. This observation is enough for us, by some - of course, very far - approximate the required distance.

The length of our arm with the arm from the fingers to the eye is probably known to us quite closely; equals - let's say - 80 cm. The distance between our pupils is, or at least it may also be known to us; suppose, that equals 7 cm. If we also know, that the breadth of the house is 20 m, that's half that size, or 10 m, is the last of the three terms of proportion, where by x we denote the searched distance:

0,07 : 10 = 0,8: x

From this proportion we find x = (0,8 • 10) / 0,07 = około 115 m.

And from what this proportion was made, it's easy for everyone to guess.