|The Wenham Binocular Prism
The Wenham prism has been one of the simplest and most successful of the early solutions to binocular vision with a microscope. First appearing around 1860 it continued as a mainstay of binocular design until the turn of the century.
The tiny prism calls for an angle of deflection of approximately 15 degrees. The prism is situated in a small drawer just above the nosepiece of the microscope. By pulling the drawer in and out, the prism can be position in and out of the line of vision. Because the design is only suitable for objectives of short focal length typically ½", 1", or shorter the prism must be pulled out of the way for longer lenses. Thus, the microscope is converted to monocular mode with lenses of ¼" and longer focal length.
The position of the prism is critical to true binocular vision. One of the best ways to test the alignment of a Wenham prism is to remove the lens barrels. Then look up the lens barrels from the nosepiece of the microscope with the objective removed. With the prism in place the image seen through both tubes should converge. If the image do not coincide, the prism will need to be moved right or left, or the nosepiece itself may need to be rotated to bring the two images together. The prism is usually pressure fit in place, or held there with a bit of shellac or wax acting as a glue.