by A. Yariv and J. E. Pearson. by A. G. Martin and F. W. Stephenson. By J.W. Barritt and E.T. Larson. by K. J. Binns and P. J. Lawrenson. by L. G. Berry and R. M. Thompson. by P. Bradshaw, and R. F. Johnson. by J. R. Green and D. Margerison. by W. G. McLean and E. W. Nelson. by I. F. Henderson and W. D. Henderson. by A. Elliott and J. Home Dickson.