I am reading Welford’s classic book on optical aberration theory (readable at https://www.scribd.com/document/593269643/W-T-Welford), and in Chapter 8, he calculates the wavefront aberration using the formula (8.1):
However, the corresponding figure does not include the primed points $A’, \bar{A’}, P’$, or mention which region has refractive index $n/n’$. Does anyone have any idea where this formula comes from?
