microscope

Objectives

mounting thread

The first to succeed in persuading microscope manufacturers to use the same threads on their objectives was the Microscopal Society of London, known after 1866 as the Royal Microscopal Society (RMS), which is the origin of the term “society thread,” by which the standard is known. See this paper. The society thread has 36 threads per inch of the 55° Whitworth form. The male thread has an outer diameter of 0.7965 inch (between 0.7952 inch and 0.7982 inch). The female thread, in the nosepiece, has a top-of-thread diameter between 0.8030 inch and 0.8000 inch.

There also exist a Japanese Standard, JIS (36 mm), and a German standard, DIN (45 mm), the distances in parentheses referring to the distance between the face of the nosepiece and the plane of focus on the stage. Both have male threads 20.1 mm diameter, 36 threads per inch, 55° Whitworth threads (so, mechanically, they fit the society thread).

 

tube length

The standard tube length, the distance from the face of the nosepiece to the surface against which the eyepiece rests, is now 160 millimeters.  The tube length was originally much longer (on the order of 250 mm), and for many years Leitz and some other manufacturers made microscopes with a tube length of 170mm. The tube length is sometimes marked on the objective.

In modern microscopy various attachments may be inserted between the objective and eyepiece, which changes the effective tube length. To avoid this an optical system employing “infinity-corrected” objectives was developed. When such objectives are used, the microscope stand must incorporate a “tube lens.” Infinity-corrected objectives are marked with an infinity symbol, ∞.

magnification

Objectives are marked with their magnification (in lieu of focal length) and numerical aperture, which is the product of the refractive index of the medium in front of the lens, such as air, water or immersion oil, and the sine of the angle between the optical axis and the most divergent ray that will make it to the image. If the numerical aperture is greater than 0.4, a condenser is needed; very high numerical apertures require the use of immersion oil between the objective and the coverglass.

Eyepieces

The RMS established a standard for the inner diameter of the tubes into which eyepieces fit in 1882, revised in 1889. The standard eyepiece has an outer diameter of 23 mm. The magnification marked is calculated using the tube length for which the eyepiece was designed; 160 mm for JIS eyepieces and 170 mm for DIN eyepieces.

Other standards

Two smaller standards are used only for lower quality student microscopes. Both employ a 139 mm tube. One uses 21 mm outer diameter eyepieces and an objective with an outer diameter of 17.5 mm with 42 threads per inch. The other standard uses 19 mm O.D. eyepieces and 15 mm O.D., 42 tpi objectives.

Slides

The glass slides on which specimens are mounted are 1 inch by 3 inches, a standard established by the Microscopal Society of London in 1839.

Resources

An excellent, highly recommended Web site that covers these topics in much more detail, with splendid illustrations and links to the major manufacturers:

http://micro.magnet.fsu.edu/primer/anatomy/anatomy.html

www.microscopyu.com

S. Bradbury.
The Royal Microscopal Society and the development of microscopal standards: 1839 – 1939.
Proceedings of the Royal Microscopal Society, volume 24 (1989).

Standards

ISO 8038-2. Optics and Optical Instruments- Screw Threads for Microscope Objectives and related nosepieces. Part 2. Screw thread type M25 × 0.75mm.

ASTM E210 (1963) Standard Specifications for Microscope Objective Threads.

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