An Elementary Course in Synthetic Projective Geometry Part 13

*122.* _Two pairs of points, __A__, __A'__ and __B__, __B'__, being given, then the point __C'__ corresponding to any given point __C__ is uniquely determined._

The construction of this sixth point is easily accomplished. Draw through _A_ and _A'_ any two lines, and cut across them by any line through _C_ in the points _L_ and _N_. Join _N_ to _B_ and _L_ to _B'_, thus determining the points _K_ and _M_ on the two lines through _A_ and _A'_, The line _KM_ determines the desired point _C'_. Manifestly, starting from _C'_, we come in this way always to the same point _C_. The particular quadrangle employed is of no consequence. Moreover, since one pair of opposite sides in a complete quadrangle is not distinguishable in any way from any other, the same set of six points will be obtained by starting from the pairs _AA'_ and _CC'_, or from the pairs _BB'_ and _CC'_.

*123. Definition of involution of points on a line.*

_Three pairs of points on a line are said to be in involution if through each pair may be drawn a pair of opposite sides of a complete quadrangle.

If two pairs are fixed and one of the third pair describes the line, then the other also describes the line, and the points of the line are said to be paired in the involution determined by the two fixed pairs._

[Figure 33]

FIG. 33

*124. Double-points in an involution.* The points _C_ and _C'_ describe projective point-rows, as may be seen by fixing the points _L_ and _M_.

The self-corresponding points, of which there are two or none, are called the _double-points_ in the involution. It is not difficult to see that the double-points in the involution are harmonic conjugates with respect to corresponding points in the involution. For, fixing as before the points _L_ and _M_, let the intersection of the lines _CL_ and _C'M_ be _P_ (Fig.

33). The locus of _P_ is a conic which goes through the double-points, because the point-rows _C_ and _C'_ are projective, and therefore so are the pencils _LC_ and _MC'_ which generate the locus of _P_. Also, when _C_ and _C'_ fall together, the point _P_ coincides with them. Further, the tangents at _L_ and _M_ to this conic described by _P_ are the lines _LB_ and _MB_. For in the pencil at _L_ the ray _LM_ common to the two pencils which generate the conic is the ray _LB'_ and corresponds to the ray _MB_ of _M_, which is therefore the tangent line to the conic at _M_. Similarly for the tangent _LB_ at _L_. _LM_ is therefore the polar of _B_ with respect to this conic, and _B_ and _B'_ are therefore harmonic conjugates with respect to the double-points. The same discussion applies to any other pair of corresponding points in the involution.

[Figure 34]

FIG. 34

*125. Desargues's theorem concerning conics through four points.* Let _DD'_ be any pair of points in the involution determined as above, and consider the conic pa.s.sing through the five points _K_, _L_, _M_, _N_, _D_. We shall use Pascal's theorem to show that this conic also pa.s.ses through _D'_. The point _D'_ is determined as follows: Fix _L_ and _M_ as before (Fig. 34) and join _D_ to _L_, giving on _MN_ the point _N'_. Join _N'_ to _B_, giving on _LK_ the point _K'_. Then _MK'_ determines the point _D'_ on the line _AA'_, given by the complete quadrangle _K'_, _L_, _M_, _N'_. Consider the following six points, numbering them in order: _D = 1_, _D' = 2_, _M = 3_, _N = 4_, _K = 5_, and _L = 6_. We have the following intersections: _B = (12-45)_, _K' = (23-56)_, _N' = (34-61)_; and since by construction _B_, _N_, and _K'_ are on a straight line, it follows from the converse of Pascal's theorem, which is easily established, that the six points are on a conic. We have, then, the beautiful theorem due to Desargues:

_The system of conics through four points meets any line in the plane in pairs of points in involution._

*126.* It appears also that the six points in involution determined by the quadrangle through the four fixed points belong also to the same involution with the points cut out by the system of conics, as indeed we might infer from the fact that the three pairs of opposite sides of the quadrangle may be considered as degenerate conics of the system.

*127. Conics through four points touching a given line.* It is further evident that the involution determined on a line by the system of conics will have a double-point where a conic of the system is tangent to the line. We may therefore infer the theorem

_Through four fixed points in the plane two conics or none may be drawn tangent to any given line._

[Figure 35]

FIG. 35

*128. Double correspondence.* We have seen that corresponding points in an involution form two projective point-rows superposed on the same straight line. Two projective point-rows superposed on the same straight line are, however, not necessarily in involution, as a simple example will show. Take two lines, _a_ and _a'_, which both revolve about a fixed point _S_ and which always make the same angle with each other (Fig. 35). These lines cut out on any line in the plane which does not pa.s.s through _S_ two projective point-rows, which are not, however, in involution unless the angle between the lines is a right angles. For a point _P_ may correspond to a point _P'_, which in turn will correspond to some other point than _P_. The peculiarity of point-rows in involution is that any point will correspond to the same point, in whichever point-row it is considered as belonging. In this case, if a point _P_ corresponds to a point _P'_, then the point _P'_ corresponds back again to the point _P_. The points _P_ and _P'_ are then said to _correspond doubly_. This notion is worthy of further study.

[Figure 36]

FIG. 36

*129. Steiner's construction.* It will be observed that the solution of the fundamental problem given in -- 83, _Given three pairs of points of two protective point-rows, to construct other pairs_, cannot be carried out if the two point-rows lie on the same straight line. Of course the method may be easily altered to cover that case also, but it is worth while to give another solution of the problem, due to Steiner, which will also give further information regarding the theory of involution, and which may, indeed, be used as a foundation for that theory. Let the two point-rows _A_, _B_, _C_, _D_, ... and _A'_, _B'_, _C'_, _D'_, ... be superposed on the line _u_. Project them both to a point _S_ and pa.s.s any conic _?_ through _S_. We thus obtain two projective pencils, _a_, _b_, _c_, _d_, ... and _a'_, _b'_, _c'_, _d'_, ... at _S_, which meet the conic in the points _a_, __, _?_, _d_, ... and _a'_, _'_, _?'_, _d'_, ... (Fig. 36).

Take now _?_ as the center of a pencil projecting the points _a'_, _'_, _d'_, ..., and take _?'_ as the center of a pencil projecting the points _a_, __, _d_, .... These two pencils are projective to each other, and since they have a self-correspondin ray in common, they are in perspective position and corresponding rays meet on the line joining _(?a', ?'a)_ to _(?', ?')_. The correspondence between points in the two point-rows on _u_ is now easily traced.

*130. Application of Steiner's construction to double correspondence.*

Steiner's construction throws into our hands an important theorem concerning double correspondence: _If two projective point-rows, superposed on the same line, have one pair of points which correspond to each other doubly, then all pairs correspond to each other doubly, and the line is paired in involution._ To make this appear, let us call the point _A_ on _u_ by two names, _A_ and _P'_, according as it is thought of as belonging to the one or to the other of the two point-rows. If this point is one of a pair which correspond to each other doubly, then the points _A'_ and _P_ must coincide (Fig. 37). Take now any point _C_, which we will also call _R'_. We must show that the corresponding point _C'_ must also coincide with the point _B_. Join all the points to _S_, as before, and it appears that the points a and _p'_ coincide, as also do the points _a'p_ and _??'_. By the above construction the line _?'?_ must meet _??'_ on the line joining _(?a', ?'a)_ with _(?p', ?'p)_. But these four points form a quadrangle inscribed in the conic, and we know by -- 95 that the tangents at the opposite vertices _?_ and _?'_ meet on the line _v_. The line _?'?_ is thus a tangent to the conic, and _C'_ and _R_ are the same point. That two projective point-rows superposed on the same line are also in involution when one pair, and therefore all pairs, correspond doubly may be shown by taking _S_ at one vertex of a complete quadrangle which has two pairs of opposite sides going through two pairs of points. The details we leave to the student.

[Figure 37]

FIG. 37

[Figure 38]

FIG. 38

*131. Involution of points on a point-row of the second order.* It is important to note also, in Steiner's construction, that we have obtained two point-rows of the second order superposed on the same conic, and have paired the points of one with the points of the other in such a way that the correspondence is double. We may then extend the notion of involution to point-rows of the second order and say that _the points of a conic are paired in involution when they are corresponding __ points of two projective point-rows superposed on the conic, and when they correspond to each other doubly._ With this definition we may prove the theorem: _The lines joining corresponding points of a point-row of the second order in involution all pa.s.s through a fixed point __U__, and the line joining any two points __A__, __B__ meets the line joining the two corresponding points __A'__, __B'__ in the points of a line __u__, which is the polar of __U__ with respect to the conic._ For take _A_ and _A'_ as the centers of two pencils, the first perspective to the point-row _A'_, _B'_, _C'_ and the second perspective to the point-row _A_, _B_, _C_. Then, since the common ray of the two pencils corresponds to itself, they are in perspective position, and their axis of perspectivity _u_ (Fig. 38) is the line which joins the point _(AB', A'B)_ to the point _(AC', A'C)_. It is then immediately clear, from the theory of poles and polars, that _BB'_ and _CC'_ pa.s.s through the pole _U_ of the line _u_.

*132. Involution of rays.* The whole theory thus far developed may be dualized, and a theory of lines in involution may be built up, starting with the complete quadrilateral. Thus,

_The three pairs of rays which may be drawn from a point through the three pairs of opposite vertices of a complete quadrilateral are said to be in involution. If the pairs __aa'__ and __bb'__ are fixed, and the line __c__ describes a pencil, the corresponding line __c'__ also describes a pencil, and the rays of the pencil are said to be paired in the involution determined by __aa'__ and __bb'__._

*133. Double rays.* The self-corresponding rays, of which there are two or none, are called _double rays_ of the involution. Corresponding rays of the involution are harmonic conjugates with respect to the double rays. To the theorem of Desargues (-- 125) which has to do with the system of conics through four points we have the dual:

_The tangents from a fixed point to a system of conics tangent to four fixed lines form a pencil of rays in involution._