Only clickable area is triangular prism on left wall near bottom.
Take any two arbitrary directions in the plane of the paper, and draw a small isosceles triangle abc, whose sides are perpendicular to the two directions, and consider the equilibrium of a small triangular prism of fluid, of which the triangle is the cross section.
This formula holds for the general case in which the base is a trapezium; the wedge being thus formed by cutting a triangular prism by any two planes.
When hardened in situ its shape is that of a right-angled, triangular prism showing five surfaces - superior, anterior, inferior, posterior and right lateral which represents the base of the prism.
Such an instrument consists of a triangular prism set with its refracting edge vertical on a rigid platform attached to a massive stand.
