The Amer­i­can Stereo­scop­ic Com­pa­ny got the idea to com­bine the ease-of-use of the Holmes stere­o­scope design with the light-block­ing ben­e­fits of the closed-box Brew­ster design and patent­ed this Stereo- Gothard Attach­ment in 1901. You could buy this attach­ment as an add-on for a Holmes-style stere­o­scope or buy an all-in-one ver­sion called The Stereo-Gothard. Both are fair­ly rare but the Attach­ment ver­sion seems to be hard­er to find than the all-in-one. This cre­ative design facil­i­tat­ed the view­ing of the semi-trans­par­ent tis­sue stere­oviews that the com­pa­ny also published.

(from: VintageViewers.com)