1. Pledge, H. T., Science since 1500 (H.M.S.O., London, 1939).
2. Thiel, R., And There was Light, 135 (Andre Deutsch, London, 1955).
3. Hall, A. Rupert, From Galileo to Newton, 58 (Collins, London, 1963).
4. Ref. 1, 61.
5. King, H. C., The History of the Telescope (Charles Griffin and Co., London, 1955).