From the New Annotated Sherlock Holmes, note 205: "Binoculars with large objective lenses, for viewing in the dark." Their objective lenses were larger to take advantage of the dilated pupil at low light primarily to improve light collection, although I've been unable to find out if the ocular lens or the entire apparatus varied in design from a basic modification of field-glasses. I've been looking for patents, with little luck, although there are a few descriptions and examples out there that are suggestive of the principle behind low-magnification opera glasses.

According to this annotated edition of Sherlock Holmes stories, they’re a particular type of binoculars used to see at night.

I was also able to find a couple articles from the late 1800s discussing the usefulness of either telescopes or binoculars for this purpose, such as the following: “...when I had erected a telescope [...] one of its effects by trial was that when toward evening, on account of darkness, the natural eye could not penetrate far into space, the telescope possessed that power sufficiently to show, by the dial of a distant church steeple, what o’clock it was, notwithstanding the naked eye could no longer see the steeple itself.”

Source: Edward S. Holden, “Observations on the Light of Telescopes used as Night-Glasses,” American Journal of Science vol 22 (Aug 1881): pp. 129-131.

Edit: I also found this listing for a pair of antique binoculars inscribed as “Night glasses,” so this is probably a decent visual of what Holmes was using. :)