Interesting is a remark to the effect that the notion of poles and polars may be extended to spheres and to certain other surfaces of the second degree. It is likely that Desargues was aware of only a few of the surfaces of second degree, many of these surfaces probably remaining unknown until their complete enumeration by Euler in 1748. Elsewhere we find Desargues’ fundamental two-triangle theorem: If two triangles, in the same plane or not, are so situated that lines joining pairs of corresponding vertices are concurrent, that the points of intersection of pairs of corresponding sides are collinear, and conversely.