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out his cooperation the book would not have seen the light of the day. I express my deep sense of gratitude towards him. My thanks are also due to Prof. M.L. Pandya, Head of the Physics Department, Modinagar, Miss. Rita Jain, Lecturer, Meerut College, Mrs. Abha Jain M.A. and Shri Murari Lal Gupta who have helped in several ways.
I shall be very grateful for pointing out errors and omissions and for giving valuable suggestions. Healthy criticism on the subject is invited.
223, Thapar Nagar, Meerut Dated 2-4 1975.
G.R. Jain
