Matthys, Robert James. "Photoelectronics for pendulums." Accurate Clock Pendulums. : Oxford University Press, 2004-06-03. Oxford Scholarship Online. 2010-01-01. Date Accessed 25 May. 2013 <http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780198529712.001.0001/acprof-9780198529712-chapter-34>.
Matthys, R.(2004-06-03). Photoelectronics for pendulums. In Accurate Clock Pendulums. : Oxford University Press. Retrieved 25 May. 2013, from http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780198529712.001.0001/acprof-9780198529712-chapter-34.
Matthys, Robert James. "Photoelectronics for pendulums." In Accurate Clock Pendulums. : Oxford University Press, 2004-06-03. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780198529712.001.0001/acprof-9780198529712-chapter-34.
Matthys RJ. Photoelectronics for pendulums. In: Matthys RJ. Accurate Clock Pendulums. Oxford University Press; 2004. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780198529712.001.0001/acprof-9780198529712-chapter-34. Accessed May 25, 2013.