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This, and related general questions, have been treated — besides the above-mentioned authors — by M. Thiesen ( Berlin.
Akad.
Sitzber., 1890, xxxv.
799 ; Berlin.
Phys.
Ges.
Verh., 1892 ) and H. Bruns ( Leipzig.
Math.
Phys.
Ber., 1895, xxi.
325 ) by means of Sir W. R. Hamilton's characteristic function ( Irish Acad.
Trans., Theory of Systems of Rays, 1828, et seq .).
Reference may also be made to the treatise of Czapski-Eppenstein, pp. 155 – 161.

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