1. The invariants and covariants of a system of two conics have been much studied² but little has been said about those of three conies. Three conics have a symmetrical invariant Ω₁₂₃, or in symbolical notation (a b c)². According to Ciamberlini³ the vanishing of this invariant signifies that the Φ conic of any two of f₁, f₂, f₃ is inpolar with respect to the third; and in a previous paper⁴ I have derived by symbolical methods a more symmetrical result, viz., if Ω₁₂₃ vanishes, then u being any line in the plane, u₁, u₂, u₃ are concurrent, where u_i is the polar with respect to f_i of the pole of u with respect to Φ_jk.