@dave123- have you tried drawing it? its impossible to draw as you see in the original one.
If you are trying do try it in a CAD software so that thickens of lines don't effect you. In a small drawing line thicknesses seem to be irrelevant but can easily hide such differences
There is no optical illusion. Its just a careful manipulation with line thickness checkout this drawing i made in CAD ( you can try for yourself)
http://sphotos.ak.fbcdn.net/hphotos-ak-snc3/hs164.snc3/19161_340546608708_687698708_5005153_549897_n.jpg