*57. Pencils of rays of the second order.* Similar investigations may be made concerning the system of lines joining corresponding points of two projective point-rows. If we project the point-rows to any point in the plane, we obtain two projective pencils having the same center. At most two pairs of self-corresponding rays may present themselves. Such a ray is clearly a line joining two corresponding points in the two point-rows. The result may be stated as follows: _The system of rays joining corresponding points in two protective point-rows has at most two rays in common with any pencil in the plane._ For that reason the system of rays is called _a pencil of rays of the second order._ *58.* In the case of two perspective point-rows this system of rays degenerates into two pencils of rays of the first order, one of which has its center at the center of perspectivity of the two point-rows, and the other at the intersection of the two point-rows, any ray through which may be considered as joining two corresponding points of the two point-rows. *59. Cone of the second order.* The corresponding theorems in space may easily be obtained by joining the points and lines considered in the plane theorems to a point _S_ in space. Two projective pencils give rise to two projective axial pencils with axes intersecting. Corresponding planes meet in lines which all pass through _S_ and through the points on a point-row of the second order generated by the two pencils of rays. They are thus generating lines of a _cone of the second order_, or _quadric cone_, so called because every plane in space not passing through _S_ cuts it in a point-row of the second order, and every line also cuts it in at most two points. If, again, we project two point-rows to a point _S_ in space, we obtain two pencils of rays with a common center but lying in different planes. Corresponding lines of these pencils determine planes which are the projections to _S_ of the lines which join the corresponding points of the two point-rows. At most two such planes may pass through any ray through _S_. It is called _a pencil of planes of the second order_. PROBLEMS *1. * A man _A_ moves along a straight road _u_, and another man _B_ moves along the same road and walks so as always to keep sight of _A_ in a small mirror _M_ at the side of the road. How many times will they come together, _A_ moving always in the same direction along the road? 2. How many times would the two men in the first problem see each other in two mirrors _M_ and _N_ as they walk along the road as before? (The planes of the two mirrors are not necessarily parallel to _u_.) 3. As A moves along _u_, trace the path of B so that the two men may always see each other in the two mirrors.