HISTORY OF BINOCULAR MICROSCOPES

As anyone who has used a microscope will tell you, it is much more comfortable to use a binocular microscope than an monocular one, especially for prolonged observations. Binocular microscopes date back to the early days of microscopy, but until 1860 when the Wenham binocular was invented, they were all either hard to use, gave poor images, required frequent delicate adjustments, and, or were very expensive to produce. One should also realize that binocular microscopes are not neccesarily stereoscopic, and many of the designs used today are not stereoscopic, including those used for high powers. I will not discuss this further as it is beyond the scope of my website.

Among binocular arrangements for the microscope, the Wenham Binocular dominated, starting from its invention in 1860, and continuing for many years thereafter, right into the first quarter of the twentieth century. In fact, in 1910, one of the best microscopes in the world, the Watson Grand Van Heurck, was still being supplied with a Wenham tube. As originally designed, the prism for the Wenham binocular was suitable only for low powers and resulted in a 50% loss of numerical aperture. Depending on the author quoted, focal lengths as short as 1/2 an inch to 1/4 inch could be employed with this prism. There was also some asymmetrical illumination of one eye vs the other which worsened with higher powers. Once a focal length as short as one quarter inch is used, Wenham's original design does not perform very satisfactorily. Eventually various makers devised improvements in the prism arrangements to allow the use of the Wenham tube with higher power objectives. Among the designs used for higher powers was that of Powell and Lealand patented in 1865. These suffered severely from unequal illumination to the right vs left eye (6:1 difference in brightness). A truly practical symmetrical binocular design, usable with all powers, was developed by German makers in the first quarter of the twentieth century and futher improved as time went on. Part of the difficulty making good symmetrically illuminated binoculars that did not reduce the numerical aperture or reduce the brightness of the image was the difficulty of maintaining alignment, suitable coatings for reflective surfaces and making complex prisms accurately. Modern Binocular microscopes these days follow the plan of Jentzschke from 1913, as originally made by Leitz, or the plan of Siedenhoft from 1924, as made by Zeiss.

The Zeiss model, which they called the Bitumi, was originally made as an attachable head for a binocular tube. The design of the Bitumi changed somewhat from when first introduced in that the original design had unequal length tubes for the eyepieces but the later version, eliminated this. The Jentzsche model as originally designed moved the upper prisms apart as the interocular distance is adjusted, changing tube length; to keep the tube length constant a mechanical arrangement was developed which is linked to the change in interocular distance, so that the eyepieces protrude in an amount varied to maintain constant tube length. In the Bitumni model, one prism rotates around the other as interocular distance is adjusted, and then the whole binocular is rotated to straighten it out. The tube length is not changed with the interocular distance in the Bitumni, but the need to rotate the binocular head after making the adjustment is an inconvenience that has made this type of adjustment less popular, albeit less expensive to produce than the Jentzsche.