They studied all these factors, and moreover the influence of differently directed attention and suggestive attitude.

If a vertical line is immediately followed by a horizontal, the two together may give the impression of one right angle.

If the time between the vertical and the horizontal line is long, first one and then the other is seen.

But at a certain length of the time interval, a new effect is reached.

We see the vertical line falling over and lying flat like the horizontal line.