Holmes Stereoscope Viewer, also known as the Mexican Stereoscope, with wooden stand

Holmes stereoscope, also known as the Mexican stereoscope, made of metal "Monarch" with a turned wooden stand

small defect in the fabric on the right side

This type of stereoscope, dating from the 1850s to 1900, was designed for viewing stereoscopic maps, which were rapidly gaining popularity. It consisted of two prismatic lenses set within a distinctive visor (which could be made of metal), as well as a wooden support to hold the stereo map. A handle at the bottom facilitated gripping the entire assembly. This type of stereoscope remained in production for a century.

It's an American invention, created by Oliver Wendell Holmes, an American poet and physician, although it's often called the "Mexican stereoscope"! Holmes didn't invent the optical principle with two prismatic lenses. But he did create a practical device with its characteristic visor, wooden stand for holding the stereo card, and handle for gripping.