The centres of circles forming a coaxal system are **collinear** 2.

A and a are called the "pointers," since they are **collinear** with, or point to, the pole-star.

The **collinear** centres are the three sets of one external and two internal centres, and the three external centres.

- the sum of the moments of the forces about each of the three points which are not **collinear** must be zero.

The origins of these four plane co-ordinate systems may be **collinear** with the axis of the optical system; and the corresponding axes may be parallel.

It is readily seen that if the optical system be symmetrical, the origins of the co-ordinate systems **collinear** with the optical axis Object J Barrel shaped Cushion shaped Distorted image FIG.

In this way he established the famous theorem that the intersections of the three pairs of opposite sides of a hexagon inscribed in a conic are **collinear**.