Screen scaling for T-illusion experiment

[From Bill Powers (950220.1340 MST)]

To Modelers, RE: T-illusion

In playing with Bruce's code, I realized that the horizontal and
vertical sizes on the screen may not be represented correctly as numbers
when doing the analysis. It's necessary to adjust the screen for a 1:1
aspect ratio so one unit of vertical size equals one unit of horizontal
size. The best way to set up the screen is to draw a square and MEASURE
its sides. Most monitors allow adjusting at least the vertical size.

If the disturbance is zero-centered, it should be added to and
subtracted from a constant size that is the same base size for the
vertical dimension.

Best,

Bill P.