Projective Geometry: Proving Existence of Centers S_1, S_2 & S_3

[Norman.Galois]

I'm doing reading course on Projective Geometry.

I was presented this question (in the textbook, not homework):

In P_2 R, let A, B, and C be points on a line L and let A', B', and C' be points on a line L'. Prove there exists points S_1, S_2 and S_3, and lines l_1 and l_2 such that projection from L to l_1 with center S_1, ...

And it continues. The remainder is not important. What do they mean by center S_1?

Thank you.

 

[radou]

Isn't it a fixed point in some projection?

 

[Norman.Galois]

Isn't it a fixed point in some projection?

That seems to make more sense. I'll try that out. Thanks!