This slider contains a Wenham Prism designed to split the image between the center and side tube. This allowed, at low magnifications, an early form of binocular vision. The right image shows how the prism splits the image.
The prism would be slid into position just above the fine focus mechanism.
Binocular tubes of the Powell and Lealand microscope consisted of one upright and one angled tube. Tube length and interpupillary distance was adjusted by a rack and pinion mechanism.