It is hardly a principle as yet.

It is rather a word used to cover all illusions which spring up when surfaces of different sizes and shapes, looked at together or successively, are misjudged with reference to one another.
Wishing to investigate this in a simple way, the following experiment was planned and carried out by Mr.B.
He wished to find out whether, if two detached surfaces of different sizes be gazed at together, the linear distances of the field of vision (the whole scene visible at once) would be at all misjudged.

To test this, he put in the window (W)[5] of the dark room a filling of white cardboard in which two square holes had been cut (S S').

The sides of the squares were of certain very unequal lengths.

Then a slit was made between the middle points of the sides of the squares next to each other, so that there was a narrow path or trough joining the squares between their adjacent sides.