Another favourite illusion is to ask a person to mark on the wall a height from the floor which would represent the length of a horse’s head: here the majority guess far too little—for a horse’s head is much longer than most people imagine, ranging from 25 to 34 inches. In a recent experiment 5 persons out of 6 under-estimated the proper height.
Here are two triangles. Which is the one whose centre is the better indicated? (It looks like A, but it is B).
Again: Out of the two straight lines C and D which is the longer? (By measurement we see they are both the same).