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127 - On the Homographic Transformation of a Surface of the Second Order into itself

Published online by Cambridge University Press:  03 May 2011

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Summary

I pass to the improper transformation. Sir W. R. Hamilton has given (in the note, p. 723 of his Lectures on Quaternions [Dublin, 1853)] the following theorem:—If there be a polygon of 2m sides inscribed in a surface of the second order, and (2m − 1) of the sides pass through given points, then will the 2m-th side constantly touch two cones circumscribed about the surface of the second order. The relation between the extremities of the 2m-th side is that of two points connected by the general improper transformation; in other words, if there be on a surface of the second order two points such that the line joining them touches two cones circumscribed about the surface of the second order, then the two points are as regards the transformation in question a pair of corresponding points, or simply a pair. But the relation between the two points of a pair may be expressed in a different and much more simple form. For greater clearness call the surface of the second order U, and the sections along which it is touched by the two cones, θ, ϕ; the cones themselves may, it is clear, be spoken of as the cones θ, ϕ. And let the two points be P, Q. The line PQ touches the two cones, it is therefore the line of intersection of the tangent plane through P to the cone θ, and the tangent plane through P to the cone ϕ.

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Publisher: Cambridge University Press
Print publication year: 2009
First published in: 1889

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