In Act I Scene I of The Merchant of Venice Shakespeare's character Salarino uses a double negative in the phrase Not in love neither?, is this grammatically wrong or was this acceptable at the time?
[Antonio is sad, his friend Salarino tries to cheer him up]
Why, then you are in love.
Not in love neither? Then let us say you are sad,
Because you are not merry: and 'twere as easy
For you to laugh and leap and say you are merry,
Because you are not sad.
I have looked around a bit and in most modern and old Romance Languages, some Germanic Languages and Old English1 double negation is simply used to reinforce a negative or simply to make it grammatically correct, so it is possible that Shakespeare intended for it to mean a negative2 - as opposed to the more modern double negative = positive. However, from the context, it would seem that Shakespeare intends this to mean the modern equivalent of 'Not in love either?'; ruling out that answer.
Alternatively, this could be litotes, which is used to understate a piece of speech or simply a classic Shakespearian mistake.
Using this as context, would this sort of double negation be common or even understood?
Would it have been grammatically correct then?
2: http://nfs.sparknotes.com/merchant/page_4.html 'translates' the Shakespeare to say You’re not in love either? backing up the negative intention view. (Thanks to @Keep these mind for raising this)