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.