In 1471, Johannes Müller asked where you should stand so that a vertical bar appears longest.
To be more precise, suppose a vertical bar is hanging so that the top of the bar is a distance a above your eye level and the bottom is a distance b above your eye level. Let x be the horizontal distance to the bar. For what value of x does the bar appear longest?
Note that the apparent length of the bar is determined by the size of the angle between your lines of sight to the top and bottom of the bar.
Please don’t give solutions in the comments. I’ll post my solution tomorrow, and you can give your solutions in the comments to that post if you’d like.
Update: See this post for more historical background.