ROSS-ZENTMAYER PATENT MICROSCOPE

HISTORY OF ROSS-ZENTMAYER MICROSCOPES

The firm of Andrew Ross dates back to the 1830's or earlier. The first known reference to him appeared in the Transactions of the Society of Arts in 1832, p 413 where it mentions his construction of a microscope for Valentine in 1831. In the achromatic era dating from about 1839, Ross soon became the foremost maker of microscopes in England. His instruments were of high quality, and although sometimes not up to the extreme standards of the small company of Powell and Lealand, his stands were more popular, less expensive and made in much greater numbers. When Andrew died in 1859, his son Thomas took over the business. When Thomas died in 1870, Francis Wenham was called upon to direct the further production of microscopes. In 1882 he designed the complexRadial microscope that often bears his name. Wenham left the firm that same year.

Wenham, primarily a marine engine and propeller engineer, also contributed greatly to early aviation theory and some of his work likely influenced the Wright Brothers. Among Wenham's accomplishments were his high pressure steam engines which were able to increase the speed of ships faster then the earlier low pressure steam models. Wenham's constributions to microscopy were also quite extensive, so much so, that, as noted above, he was chosen to help guide the designs of the Ross company once Thomas Ross had died.

One of the major pursuits of microscopists of the last quarter of the nineteenth century (and before), was a desire to increase contrast and resolution. Before Abbe's theories were popularized, one of the ways this was accomplished was via the use of oblique illumination via an angled light source below the stage. From this, it followed that the greater range of choices of oblique illumination, and the more extreme it could be, the more likely that a suitable arrangement could be achieved for optimal resolution and contrast of fine features of microscopical objects, especially diatoms. With the small diameter condensers of the time, extreme degrees of oblique illumination could only be achieved by illuminating from an angle rather than directly from below with the light parallel to the optical axis and perpendicular to the stage. Swinging substages that did not swing in the axis of the slide existed long before, and one that rotated in the axis of the specimen was originally introduced years before, for example in the early 1850s, exemplified by the Grubb Sector Microscope, but the idea did not then take off. The first person to popularize a method in which the swinging substage for this purpose rotated through the axis of the specimen and therefore did not have to be refocused as it swings, was the famous American microscope maker, Joseph Zentmayer. By having its axis of rotation at the plane of the slide, the focus of the condenser was maintained throughout the range of obliquity. This feature soon became very popular and Francis Wenham quickly became one of its proponents in England. Additional important characteristics of this arrangement are the fact that the condenser had to be narrow enough to allow this without getting blocked by the edge of the stage, and also that the stage was thin enough so as not to obstruct illumination from an extremely oblique angle. These became the essential features of the Ross-Zentmayer microscopes.

The English patents for these microscopes were applied for in 1876 and 1877. These models were first exhibited to the RMS in 1878(see JRMS 1878 p 163 and 197-199) but there was no illustration of them in the JRMS that year. The first illustration of the Ross-Zentmayer model that we are aware of appeared in the January 1880 Ross Catalog, showing the microscope on an English curved claw foot, with concentric mechanical stage controls (left). The model was referred to there as the New Patent Microscope. It was stated from the first that there were 4 different versions, Nos 1 to 4, and that the curved claw foot could be ordered solid or with folding feet to fit in a more condensed case (with all its accessories). The folding curved English claw foot example shown below has a serial number of 5029 which dates it to about 1880:

The Ross Zentmayer was first shown in the JRMS later in the same year(1880) on page 704, but this time with the lyre-shaped foot. By the time the 1882 catalog came out, the claw foot (folding or not folding), and the lyre-shaped foot were both offered. As shown below, the Ross-Zentmayer models also appeared in the 1882 and January of 1883 catalogs. As noted above there were four models. Number 1 was the largest, number 2 slightly smaller, and number 3 smaller yet. The smallest and least complex was the number 4. All the Ross-Zentmayer stands could be ordered with any one of the three different variants of the foot.

According to an advertisement in the JRMS of 1898 and the Annual Review of Microscopy of the same year, Ross had introduced continental stands by that year, but, as noted above, continued to offer the Ross-Zentmayer and Wenham Radial microscopes at that time. By the early 20th century, with widespread knowledge of Abbe's work, Ross adopted the continental form of microscope and the Ross Zentmayer model was abandoned.

Not only did Wenham come up with the Ross-Zentmayer models, but an even more complex stand which became known as the 'Wenham Radial' or the 'Ross Radial' a rare microscope which was already obsolete when first produced. Wenham applied for the patent of the Radial in 1881 and it was first produced in 1882. The inclusion of the swinging substage on microscopes soon faded as the newly introduced Abbe condensers with their much larger lenses appeared. The addition of a swing-out ring which could hold filters or metal inserts with various openings and a moveable iris diaphragm, or both, allowed oblique lighting to be easily achieved in these large lens condensers, without the swinging substage. Either asymmetrical stops could be inserted in the swing-out ring, or in some examples, the iris could be offset to the edge of the optical path of the condenser so as to direct the light obliquely.

Furthermore, the Ross-Zentmeyer design had major flaws in the design of their fine focus mechanisms(as did Zentmeyer's own), which also limited the popularity of the Ross-Zentmeyer stands. The design was flawed in three ways. First, the long lever focus employed a flimsy lever which was not rigid enough, leading to backlash. Secondly this flimsy lever was not rigid enough to regularly overcome the friction of the dovetail for the fine focus mechanism. Lastly, as the brass in the dovetail wore with use, there was no way to adjust the friction, so the tube could wobble to the right or left. Multiple attempts to improve the situation included the use of rollers, and/or washers, but these were met with very limited success. Others, most notably Watson*, had success with the long-lever fine focus. It is of some interest that with the exception of the small Student Tripod Microscope the only microscopes offered in the 1883 Ross catalog were the Radial and the various iterations of the Ross-Zentmayer. By 1895 the Ross company was still offering the Ross-Zentmayer model but had also started to supply stands with continental limbs as noted in the Transactions of the American Microscopical Society of that year. By 1900, the age of the swinging substage had come to an end.

In 1882, the first year of production of the 'Wenham Radial,' Francis Wenham became disenchanted with microscopy, having been embarassed by his erroneous and widely publicized disagreements with Robert Tolles. After 1882 Wenham no longer directed microscope development at Ross. The Ross-Zentmayer stands quickly gave way to more modern continental stands, but by 1906 Ross was no longer making microscopes at all.

*Watson found ways to deal with these fine focus issues by making the dovetails adjustable for wear and using a much beefier lever. Their fine focus mechanism was successful for many decades, most famously on their Van Heurck Models, which were also updated with substages that did not swing, but accomodated the much larger condensers of the time.