THE system of conies having double contact with two conies includes as special cases the two systems of conies passing through four fixed points and touching four fixed lines, respectively. These latter result from making the fixed conies break up into factors, in the first case, in the tangential sense, and in the second case directly. Thus the general case includes and combines together into one whole the properties of the two special cases. Now I propose to consider the locus of the foci of the general system. This must include as special cases the known loci in the case of conies through four fixed points and touching four fixed lines. The first is a curve of the sixth order, which, when the points are coneyclic, as has been shown by Professor Sylvester, breaks up into the two circular cubics having the points for foci, and the second is a circular cubic having its double focus on itself and passing through all the intersections of the four lines. Now foci, being the intersections of tangents drawn to the curve from the imaginary points at infinity, are most easily treated by tangential coordinates, which I consequently use, the equations being precisely the same as the direct ones ; viz. as given by Salmon, a conic having double contact with S, S is, if $ JcS = BF;