vanishing point

These formulae . . . give for the vanishing point of a set of parallel lines whose common azimuth is α and declivity δ: η=d tan α, ζ=d tan δ sec α.

At 8h 35' 4", the least dent possible, quite black, appeared in the sun's limb. And at 8h 35' 6", the limb was restored to its perfect form, there having been a small trembling light, between the narrow watery border of Venus and the vanishing point of contact in the sun's limb for these two seconds of time.