Few Japanese people probably know this side of Emperor Meiji, who left behind poems like this.
Literal
This-kind-of poem [object-を] leaving Meiji-Emperor [possessive-の] one-side [object-を] knowing Japanese [topic-は] few is-not [wonder-だろうか].
The rhetorical construction ~のではないだろうか ('isn't it that...? / I wonder if...') is a characteristically Japanese way of making an assertion sound like humble contemplation. こんな歌を残している modifies 明治天皇 as a relative clause. 一面を知っている (know one side of) suggests hidden depth.