An Elementary Course in Synthetic Projective Geometry

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An Elementary Course in
Synthetic Projective Geometry

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Projective Geometry by Lehmer, Derrick Norman
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Title: An Elementary Course in Synthetic Projective Geometry
Author: Lehmer, Derrick Norman
Release Date: November 4, 2005 [Ebook #17001]
Language: American
Character set encoding: ISO-8859-1
***START OF THE PROJECT GUTENBERG EBOOK AN
ELEMENTARY COURSE IN SYNTHETIC PROJECTIVE
GEOMETRY***

An Elementary Course in Synthetic Projective Geometry
by Lehmer, Derrick Norman

Edition 1, (November 4, 2005)

PREFACE
The following course is intended to give, in as simple a way as possible,
the essentials of synthetic projective 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 has been a slight
smoothing of the road in some places. Especially will this be observed
in the chapter on Involution. The author has never felt satisfied with the
usual treatment of that subject by means of circles and anharmonic
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 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. He 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. Thus, 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
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. The author is aware of the importance of work of this sort,
but he does not believe it is possible at the present time to write a book
along such lines which shall be of much use for elementary students.
For the purposes of this course the student should have a thorough
grounding in ordinary elementary geometry so far as to include the
study of the circle and