These are known as “military glasses” and “field glasses”. This is to distinguish them from prism binoculars which give better magnification in a more compact instrument.
This specimen is in the style of the mid-nineteenth century. They are based on the Galilean telescope which is a simple telescope with a convex lens as the objective lens (the one at the far end) and a concave lens as the ocular (the lens close to your eye).
They can lengthen to ten inches and compress to eight inches. The longer this is extended, the more magnification that results. Of course the field of view has to be limited with increasing magnification. The optics still are clear in this pair.
The distance between the oculars is set by rotating both barrels along the long axis.
As these are sold brass, they are fairly heavy but rugged. The estimated weight is over one kilogram (2+ pounds).