Since a ruled surface contains only ∞ elements, this theory is practically the same as that of curves.
The ruled surface is, moreover, concave, and hence brings the spectrum to a focus without a telescope.
The degree of a ruled surface qua line geometry is the number of its generating lines contained in a linear complex.
In the general case of a singly infinite system of lines, the locus is a ruled surface (or regulus).
We have occasionally met with leaves having a lined or ruled surface, giving most beautiful diffraction colours.