The focal ring is also known as the reference circle.
2.
A "'reflection "'is a inversion as the radius of the reference circle increases without bound.
3.
For a circle not passing through the center of inversion, the center of the circle being inverted and the center of its image under inversion are collinear with the center of the reference circle.
4.
It follows from the definition that the inversion of any point inside the reference circle must lie outside it, and vice versa, with the center and the point at infinity changing positions, whilst any point on the circle is unaffected ( is " invariant " under inversion ).
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