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Conics(3) → This page is seriously in need of revision ←
The Fundamental Theorem reveals a set of lines that envelope a curve, 

This.... 
.....Is this in perspective.... 

....and these are just two views of a “rocking” coneandcuttingplane ensemble.
Click 'n' drag the bold dashed line on the left to rotate the cone and its cutting plane around the “horizontal” axis. Note how the lines from the cone's apex tangent to the circle (representing the ‘horizons’ of the cone's surface) vanish when the cone's apex appears to pass inside the conic made by, and lying on, the cutting plane— Because the cone's surface then has no visible ‘horizons’ (sometimes styled, ‘cuspidal edges’) , the tangents are imaginary. The black line, a, represents the “lineatinfinity” for the planes of rotation. Click 'n' drag point A to move this line left or right to change the perspective. —and/or start the animation. 

Some General Features of Conics →  
On the left is an interactive version of the pictures above. You may "click 'n' drag" various points to adjust the conic, and to change the relationship of the conic's centre (pole) to the "line at infinity" (polar), and you can drag point R on the polar to rotate the whole thing round the pole, or centre. (Is it not curious that R is rotating, even while moving in a straight line? This is characteristic of Polarity.) You may also start or stop the turning using the animation control. The little red slider adjusts the rate of rotation 

On the right we see, in perspective, a plane holding conics rotating on a fixed, thin, vertical blue line, and taking the conics round with it, rather than, as above, lines of the conic turning on a fixed point on a fixed plane. (It is a variation of the rocking cone, above.) The bold blue line could, but need not, represent the For fun, part of a cone has been ‘constructed’ on the bold conic, and given a “transmission rod” on its axis, all to simulate a radar dish in rotation—but also to bring out a serious point, which is that nonphysical, nonEucliean geometry may also apply to physical reality, such as a radar dish. And we see a condition for it to apply:— 

For we recall that the axioms of incidence are qualitative, not quantitative, and so do not define distance, or indeed countable units of anything, so all references that depend for their validity on infinity understood as as sum of discrete distances (that is, as a quantity), are "interlopers". They are wrong, and wrongly placed, because they rely for their definitions on both quantity and distance. They are parvenu, and alien, and to be extirpated. Then, especially, references invoking parallelity cease to have any kind of meaning (if, indeed, they ever did), and the "line at infinity" (the one found to be carrying two pairs of conjugate points, and also referred to in the discussion of the radar dish) is just an ordinary line. And, most notably, we must recognise that Intermezzo, entitled “Seeming and Being.” (skip↓) That the two conics appear different may seem just a matter of viewpoint Moreover, we might wish to assert that a view is only a copy (that is, a projection), not the original, But—and is it a big ‘but’—this is to imply that the original is, in some sense,
then any one of these conics could serve as an ‘original’. To put it another way, In fact, we must conclude that It must follow that, when projecting, instead of copying some absolute, invariant original, For example, all the properties of incidence with lines and points determine how all conics look—just as long as they remain unseen! For it would very much seem Here we have the issue, I believe, that so exercised Einstein In our case, we know that the base elements of geometry are forever invisible, but we are, notwithstanding, entirely sure that the axioms of incidence, It is therefore a vitally interesting puzzle 

So there is no "niche" within the axioms for curved items such as nondegenerate conics, or the surfaces of (for example) cones, or eggs, or buds, so Curvature is not axiomatic. If Curvature must instead be specified by those axioms that are actually available, And what is more, Motion, 

Nevertheless, if we allow the intuitive notion of Motion to stand for now, a tangent and its contact point can be said to represent or specify a curve in a single place as a combination of two axiomatic elements—a line (wanted for its orientation), and a point (wanted for its position). By the axioms of incidence, these elements can combine (that is, be mutually incident) in only the following two ways—
Then, to represent another place on the curve, the whole combination must move, and, by the axioms again, while moving,
Now, as there is no axiom of curvature, on a move the point is free to go anywhere in that point's line, and that line is free to rotate at random in that line's point. Let us look in some detail at how we want it to move. 

Suppose we draw an arbitrary curve. Drawing it, we know that it is continuous, because we never lift our pen from the paper, so there is ink—points—to be found everywhere and anywhere on the curve^{3}. We see that it is the act of drawing, and not the axioms, that selects the points that lie on such a curve. This is what makes it "arbitrary". On this curve, we choose two distinct points, T_{1} and T_{2}, and draw tangents, t_{1} and t_{2} respectively, to the curve at these points. They meet in a point, X. Also, we connect points, T_{1} and T_{2}, with line, x. We think of these tangents and their points as two, successive (the first and the second) dispositions of a combinationinmotion, and understand that, in the course of that motion, the tangentofthecombination will turn continuously around the pointofthecombination in contact with the curve, while that point moves along that tangent, taking point T from position T_{1} to position T_{2}, and rotating line t from orientation t_{1} to orientation t_{2}. 
"The Anatomy of a Plane Curve" animation 

We note that the kind of the curve must depend greatly on how this line and its point move, or are moved, especially in regard to their relative rates of motion. We have here a major “paradigm shift”, for, up to now, we have not needed to measure, because Projective Geometry, as an "incidencepreserving" system, does not define sizes of anything. But our arbitrary curve did not arise from the axioms of incidence. It arose from a free "act of drawing." 

Measurement, then... We need sizes, and units, after all... Projective Geometry cannot provide absolute size, but it can provide regular intervals; that is to say, intervals which are equal with respect to the manner of their making ("regulation"). These intervals are geometrically indistinguishable, one from another, and therefore can be identical and countable units, and so be the basis of a metric, or system of measurement. Two such measures working together on the plane define a coordinatesystem: the socalled Cartesian system (the familiar, squared “graphpaper”, or chart, with axes at right angles) is an instance. That same graphpaper viewed in perspective is another. Both of these styles are illustrated on the right. 
The old, familiar Cartesian Coordinates  

In the first of these two instances, the two “endpoints” or “invariants ^{5}” belonging to each of the two scales lie together and at the infinite. The second case is the same, except that that line at infinity, linking these endpoints, is in plain sight, perhaps the "sight" of a camera, such as might be used to take a photograph of the above graphpaper. (We know by now that we need to be geometrically circumspect about this "infinity"  because geometry is blind to it  but let us, provisionally, define it physically—that is, call it a place at which, say, the effect of a sun's gravity, or the intensity of the electric field due to a point charge, falls to zero. I think we can agree there might actually be such places, that we could probably detect or infer them, and that they may be, for corporeal beings like us, physically unreachable. Given that the sun, or the charge, is reachable with finite effort, we could then decide, provisionally, that
But there is no absolute fiat concerning the orientation of the axes with respect to each other. They need not be at right angles—indeed, the pure geometry cannot specify this or any other angle. To serve as axes, the axes need only be distinct (apart from their meeting place), and bear scales. Now, the numbering of intervals on a scale may begin at any of its intervals. Where there are two or more axes with scales, it is often convenient (though not absolutely required) to begin the counting for them all at the meeting place of their axes, as the "Origin" of coordinates. The zeromarks of all the scales lie at this origin. 


These systems assign numbers to places, at the rate of one number (or "ordinate") per axis per place (so that the members of a numberordering are thus "coordinates"), so we might be able to describe our arbitrary curve in terms of some "ordered" series, or set, of assignments—and if we can do this, then we have "measured (metricised) " the curve, and it is no longer entirely arbitrary! 

Moreover, if we specify a continuous curve, then we must have that to every number we choose from one axis, there will correspond a definite number, or numbers, determined from the curve, on the other axes^{4}. And each of these numbers can take any value (including infinite), and have either sign, or no sign (zero and infinity are unsigned, as each marks a transition from a segment of one sign to a segment of the other). 

A Question of Appearance One often reads that an ordered assignment, supplied by an equation such as y = 1/x (for which there is exactly one y for each x), is discontinuous at x = 0, because it appears that, for this x, y has two values of opposite sign; +∞, and ∞. But it is just that—appearance. In fact, as noted above, numerical infinity is unsigned. There is therefore just one value of y for x = 0, namely ∞, not two values, and so y = 1/x is everywhere continuous. When plotted on a standard cartesian chart, this equation produces a hyperbola (which has the chart axes as its asymptotes), that indeed appears to be doubly discontinuous, once for when x = 0, and again when y = 0. But if the curve is plotted on a perspective version of the cartesian chart, then it can instead appear as an ellipse crossing the line joining the points (0,∞) and (∞,0) [the "line at infinity"] without discontinuity, twice, and we see why the axes are asymptotes! y = 1/x is an algebraic equation, or "function". Algebraic functions must have at least one solution (never none), and may have many, but they are never discontinuous. Below the perspective plot of y = 1/x, there is a perspective plot of y = x^{2}, which, on a "standard" Cartesian chart, gives a parabola, but, as here, can appear as an ellipse. In any case, it is a closed conic, tangent to the line y = 0, and tangent to the line y = ∞, when x = 0, and once again, though appearance might suggest that a parabola is a curve closed only at one "end", now we see that it is entirely closed, "begins" at zero, and "ends" precisely at infinity.
We might say, very loosely, that—

This interactive shows that a parabola, with equation y = x^{2}, can appear as an ellipse with a major axis of precisely infinite length! 

We see that all nondegenerate conics, including the hyperbola and the parabola, are closed, and so divide their planes into two regions, both of which are enclosed by the conic. This "enclosing" property of the conic since it admits the possibility of a line travelling entirely in only one of the regions, such that it appears to "miss" the conic. Yet again this is appearance only, for the conic's relationships to lines and points extend into both of the regions it encloses, and such a line, though apparently missing the conic, has those relationships (namely, the socalled “imaginary” connections) pertaining to the region in which it happens to be. 
For example, the interactive above depicts the transformation of a line into itself through an interaction of the line with a conic  namely the black conic with the yellow point. Other conics are revealed, generated by this interaction. Drag either the black or the white line, or drag both, to cut the original black conic  or not. You will see elements of the transformation appear, as they become "real", or disappear, as they become "imaginary". But the generated conics remain real, whatever the states, real, or imaginary, of their generating elements. 

Now, deploying these ideas, let us plot a circle of unit radius, with centre on the origin, and equation y2 = 1  x2, on both styles of chart. 

It will be appreciated that the "infinity" referred to here in, "line at infinity", is of the numerical variety. 

It is worth bringing out that straight lines, such as tangents, with equal slopes (gradients), being "parallel", when plotted on either of these coordinate styles, must, by the axioms of incidence, all meet in the same point on the line at infinity. If in fact they are tangents to curves, then their slopes must be the first derivatives (dy/dx) of the curves they contact at the points of contact. The line at infinity is thus an axis, too, of a kind, but the scale on it denotes rateofchange, not position. 

If two such parallel lines are tangent to a circle (as above at contact points A and B, for example, or as on the right), then the chord through their contact points must always pass through the centre of the circle. Here we have the fundamental reason why the geometry of pole and polar works.


So, just by "running these findings backwards", we can say:
With things generalised like this, we speak of the Centre as the Pole, and of the corresponding Line at Infinity as the Polar.

It may come as a surprise that when the line at infinity cuts the conic in two real points, the centre of the conic appears to lie outside the conic, but when the line at infinity misses the conic (and therefore cuts the conic in two imaginary points) the centre appears to lie inside the conic. This might give us pause! See the discussion below.
run / stop animation In fact, "inside" and "outside" are undefined: all we have is a closed curve, dividing the plane into two regions, both of which are enclosed by the curve. We might wish to adopt the convention that the region containing the centre is "inside" the curve, but, if we do, we should keep in mind that this is an artifact with conventional meaning only, to which geometry is blind. 

So here, at last, is In the diagram on the left, Point T projects point E on the conic into point D on line l via line c. The polar of point D (passing through the pole of line l) and the polar of point E must both pass through the same point—the pole of line c—on the tangent to projecting point T.


In the second diagram, exactly this construction is repeated, and stepped forward several times, now using two projecting "pointsT" in the conic, giving a range of "pointsD" in line l, corresponding to a range of "pointsE" in the conic.
The polars of pointsD, since they all pass through the same point (i.e., the pole of line l), must produce two ranges of intervals with the same crossratio in the two tangents at pointsT. And, as we see, Thus, Four lines projecting a constant cross ratio from one line to another 

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