This fact has been stated previously by Heymans; but by the above results we can understand the nature of this feature more fully than can be done by those he obtained.

The above figures show that this is a feature of both the feather-head and arrow-head figures, although the character of the variation in these figures is somewhat different.

The curves of the above figures, especially those which represent the results of observations with the feather-head figures, indicate two interesting features of this maximal illusion.

The one feature is that the smaller the angles, the shorter is the length of the arms at which this maximal illusion occurs.

Thus, if we look successively at the curves corresponding to angles of 18, 36 and 54 degrees in Figs.