Why did Latin prefix a(d)- to vis(um)?

1. What does the Latin prefix de mean?
2. What is the Latin word ad?

What does the Latin prefix de mean?

from Latin, from dē (prep) from, away from, out of, etc. In compound words of Latin origin, de- also means away, away from (decease); down (degrade); reversal (detect); removal (defoliate); and is used intensively (devote) and pejoratively (detest)

What is the Latin word ad?

Etymology. From the Latin preposition ad (“to, towards”), in turn from Proto-Italic *ad, from Proto-Indo-European *h₂éd (“near, at”).