The feminine may discard a vowel which appears in the masculine; as
in--

actor--actress
master--mistress
benefactor--benefactress
emperor--empress
tiger--tigress
enchanter--enchantress

_Empress_ has been cut down from _emperice_ (twelfth century) and
_emperesse_ (thirteenth century), from Latin _imperatricem_.

_Master_ and _mistress_ were in Middle English
_maister_--_maistresse_, from the Old French _maistre_--_maistresse_.


31. When the older _-en_ and _-ster_ went out of use as the
distinctive mark of the feminine, the ending _-ess_, from the French
_-esse_, sprang into a popularity much greater than at present.

[Sidenote: _Ending_ -ess _less used now than formerly._]

Instead of saying _doctress_, _fosteress_, _wagoness_, as was said in
the sixteenth century, or _servauntesse_, _teacheresse_,
_neighboresse_, _frendesse_, as in the fourteenth century, we have
dispensed with the ending in many cases, and either use a prefix word
or leave the masculine to do work for the feminine also.