To load the device, the front pan­el is opened and a tray of 25 glass slides can be insert­ed. By turn­ing a side lever, one of the images is pushed upwards from the tray by two met­al fin­gers from below, allow­ing it to be viewed. Con­tin­u­ing turn­ing the lever returns the slide to the tray, which then shifts slight­ly for­ward to posi­tion the next image. A pull knob on the side allows the tray to be advanced or reversed rapid­ly, mak­ing it pos­si­ble to jump direct­ly to a spe­cif­ic image.
(from: Com­pendi­um of Stereoscopes)

Fur­ther read­ing: Stere­oscopy His­to­ry Series — Mack­en­stein (A. Ruiter)