This is a cross-eye stereo pair: To view the 3D image, pick a distinct feature of the picture and its match in the opposite image. Hold your hands with fingers straight, pointing up, about a quarter of the distance from your eyes to the screen so there's a "window" between them that's the same apparent width as one of the images. Cross your eyes until the two images overlap. You should then be able to comfortably see, in the center of your field of view, the merged image with a clear perception of depth in the picture.
The effect is much easier to see in the larger image of the slide show - or when viewing a print from Limited Edition Photo.

To date, this is the second oldest stereo pair image (by image date) I have published.