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Derrick Norman Lehmer, "An Elementary Course In Synthetic Projective Geometry"

AN ELEMENTARY COURSE IN SYNTHETIC PROJECTIVE GEOMETRY BY DERRICK NORMAN LEHMER. COPYRIGHT, 1917, BY DERRICK NORMAN LEHMER ALL RIGHTS KK8KKVK1 PRINTED IN THE UNITED STATES OF AMERICA 226.7 GINN AND COMPANY PRO PRIETORS BOSTON U. S. A. PREFACE The following course is intended to give, in as simple a way as possible, the essentials of synthetic protective geometry. While, in the main, the theory is developed along the well-beaten track laid out by the great masters of the subject, it is believed that there lias been a slight smoothing of the road in some places. Especially will this be observed in the chapter on Involution. The author lias never felt satisfied with the usual treatment of that subject by means of circles and an harmonic ratios. A purely projective notion ought not to be based on metrical foundations. Metrical developments should be made there, as elsewhere in the theory, by the introduction of infinitely distant elements. The author has departed from the century-old custom of writing in parallel columns each theorem and its dual. He has not found that it conduces to sharpness of vision to try to focus his eyes on two things at once. Those who prefer the usual method of procedure can, of course, develop the two sets of theorems side by side the author has not found this the better plan in actual teaching. As regards nomenclature, the author has followed the lead of the earlier writers in English, and has called the system of lines in a plane which all pass through a point a pencil of rays instead of a bundle of rays, as later writers seem inclined to do. For a point considered iii iv PROJECTIVE GEOMETRY as made up of all the lines and planes through it he has ventured to use the term point system, as being the natural dualization of the usual term plane system. He has also rejected the term foci of an involution, and has not used the customary terms for classifying involutions hyperbolic involution, elliptic involution and parabolic involution. lie has found that all these terms are very confusing to the student, who inevitably tries to connect them in some way with the conic sections. Enough examples have been provided to give the student a clear grasp of the theory. Many are of sufficient generality to serve as a basis for individual investigation on the part of the student. Tims, the third example at the end of the first chapter will be found to be very fruitful in interesting results. A correspondence is there indicated between lines in space and circles through a fixed point in space. If the student will trace a few of the consequences of that correspondence, and determine what configurations of circles correspond to intersecting lines, to lines in a plane, to lines of a plane pencil, to lines cutting three skew lines, etc., he will have acquired no little practice in picturing to himself figures in space. The writer has not followed the usual practice of inserting historical notes at the foot of the page, and has tried instead, in the last chapter, to give a consecutive account of the history of pure geometry, or, at least, of as much of it as the student will be able to appreciate who has mastered the course as given in the preceding chapters. One is not apt to get a very wide view of the history of a subject by reading a hundred PREFACE v biographical footnotes, arranged in no sort of sequence. The writer, moreover, feels that the proper time to learn the history of a subject is after the student has some general ideas of the subject itself. The course is not intended to furnish an illustration of how a subject may be developed from the smallest possible number of fundamental assumptions...

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