The purpose of this research is to grasp the illusion tendency of reality Arched bridge's side form, and we also confirm Arch form by changing to three centers circular and parabola.
We chose constant sum method of the experiment.
As the results, we can see the overstimated in most of the Arched bridge in any types.In these three types of Arch form, the parabola is the most overstimated and in the original (do not change the Arch form), illusion is hardly confirmed.
The amount of illusion is more than expected.