Nonius Marcellus, Compendium of Doctrine, Vol. II

Then the color of the rainbow appears in black clouds.

Fors chance and Fortuna Fortune differ in this. Fors is a temporal accident, [while] Fortuna is a goddess herself. Accius in Astyanax:

Repeat in
Which parts (for I wish to hear,
If I choose [him]) and in what manner he was captured,
Whether by chance or by Fortune found.

The same in Andromeda:

Many, woman, have increased evils for themselves in evils with an inequitable mind,
For whom nature has been more harmful than chance or fortune.

Lucilius, Satires, book XIII:

To whom fortune and fate brought an equal place by chance.

The same in the same:

Or by chance and fortune entirely to win in war:
If by chance and rashly entirely, what then regarding honor?

Varro, letter to Fufius: you will say:

O Fortune, O Fors Fortuna Chance/Fortune, with what conveniences
[Have you burdened] this day!

Ferus wild/fierce and ferox courageous/ferocious have this distance. Ferus is cruel; ferox is brave. Accius in Tereus:

A new stranger with a bold spirit rushes into the middle, ferox courageous.

Sallust in the Jugurthine War: He, with a ferox courageous spirit, denies that he fears the Numidian, who had been routed so often. — Virgil, Aeneid, book II: