Hans H:

Joshua Zucker:

...

I connected them to form a black rectangle and then it looked like a rectangle (they looked the same length). It seemed like the "smaller" one stayed the same size when I took those perpendicular lines back off, while the "longer" one seemed to jump to a bigger size when they disappeared.

I've played around with this a bit as well. The illusion seems stronger to me when the lines are vertical:

http://chesswanks.com/pot/TwoLines.jpg

If my jpg conversion maintained my thinner lines' thickness, the "longer" line also appears darker.

The darkness business also seems backwards. But Hans's rendition makes me wonder if the length business is just a case of the classic \ / \ / /|\ | / | \ | | | | | | | \ | / | \|/ | / \ / \ --rwg