9. Abh. der Ges. der Wiss. zu Göti., 13, 1868, 1-20 = Werke, 2nd ed., 272-87. An English translation can be found in W. K. Clifford’s Collected Mathematical Papers. Also in Nature, 8, 1873, 14-36, and in D. E. Smith, A Source Book in Mathematics, 411-25.