When the word either is connected immediately with the copula of a proposition, it is, if not a true conjunction, at least a part of a Conjunctional periphrasis.
"The English Language"
Robert Gordon Latham
Now, this superadded power is rather adverbial than Conjunctional.
"The English Language"
Robert Gordon Latham
Such may fairly be considered to be the case in the three examples just adduced; especially as there is also the secondary influence of the Conjunctional character of the word either.
"The English Language"
Robert Gordon Latham