DESCRIPTION OF THE BITUMI BINOCULAR:
The binocular tube replaces the draw tube by simply inserting in its place. Interocular distance is adjusted by moving either or both of the sides, with a graduated scale indicating the distance and making it easier to reset to the same distance.

HISTORY OF THE BITUMI BINOCULAR HEAD

Most binocular heads of the nineteenth and early twentieth centuries effected a change in tube length as the interocular distance was adjusted. In the 1920's Siedentopf developed a binocular head in which changing the interocular distance did not result in a change in tube length. This was important at high powers because objectives are designed for specific tube lengths and distortions are introduced if the tube length is changed. Although the early workers compensated for this with the use of correction collars on their objectives, eliminating this problem in the first place simplified higher power work. The original Bitumi used a system of pivoting eyepieces and had unequal eyepiece tube lengths because the prism for the right eye and left eye were stacked one on top of the other. The later version, seen here, compensated for changes in interocular distance by moving the prisms at the same time as the interocular distance was changed. The Bitumi head accompanying this instrument dating to after 1924, was a later version than the original, and also a later addition to this microscope. An example of the original version of the Bitumi can be seen on Alan Wisner's website here.

The author is greatly indebted to James Solliday, president of the Microscopical Society of So. California, as well as Alan Wisner, author of Antique-microscopes.com, for pointing out much of the historical and technical details, and helping to sort out the exact date of my microscope.