Deflection of Light by the Sun

[nanjo]

TL;DR

Which three points did Eddington use to define the deflection angle of 1.75"

I'm having difficulty finding the definition of Eddington's deflection angle. Can anyone tell me precisely which three points were used to define the deflection angle of 1.75" confirmed by Eddington's experiment in 1919?
John

 

[renormalize]

TL;DR: Which three points did Eddington use to define the deflection angle of 1.75"

Can you better describe the points you're asking about by making reference to the diagram below?

(from Shadow of the Moon and general relativity: Einstein, Dyson, Eddington and the 1919 light deflection)