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1873.] Mr. S. Roberts on Parallel Surfaces. 219 and there is no other such system containing y. The order of the condition for the coexistence of the last system is m (TO-1). The order of the parallel is therefore that is to say, it is equal to twice the number of normals which can be drawn from an arbitrary point to the primitive. * It seems probable, geometrically, that tho spinodo torse of the parallel is tho parallel of the spinode torse of the primitive; and in like manner, that tho nododoi:10.1112/plms/s1-4.1.218 fatcat:kcs562jnmbd2zealzaagg5oigq