Kindle No. 885 > the second half of the 1st paragraph
Kindle version (typo)
... if either $^l = f_1$ or $^l = f_2$. ... the intensity in a focus, $^l = f_1 = f_2$.
printed book (correct)
... if either $l = f_1$ or $l = f_2$. ... the intensity in a focus, $l = f_1 = f_2$.