Gossip meeting “Objectives and NA”
Tuesday 10th August 2021
Quekett members can watch a video of this meeting.
Phil Greaves introduced this Zoom meeting with a PowerPoint presentation that included lots of brass objectives (mainly Zeiss (achromatic and apochromatic) and Watson (Parachromatic, Holos and apochromatic) from his collection, a ‘Jackson’ Tubelength Corrector, a Beck objective with a Lieberkühn reflector, modern infinity-corrected Leica objectives that need a wide body and a thread larger than RMS, water immersion objectives, incident light objectives (Leitz Ultropak), reflecting objectives (Spectra-Tech and Beck), a McCrone Dispersion Staining Objective, and Watson Fizeau and Bausch & Limb Mirau interference objectives. Phil also showed Ainslie, Cheshire, Beck, Abbe and Metz apertometers (for measuring NA), and Watson Para and Zeiss oil immersion objectives that had been cut open to show their structure. Phil also also showed some formulae and diagrams to help us understand numerical aperture (NA). You can see his slides here:
Click the arrows to move through the slides. Click the symbol at bottom right for a larger version.
Pam Hamer gave an interesting presentation on apertometers, which are used to measure the NA of objectives. You can see her slides here:
Click the arrows to move through the slides. Click the symbol at bottom right for a larger version.
Here are links to the two papers referenced in Pam’s presentation:
- Cheshire, F. J., Two simple apertometers for dry lenses, Journal of the Quekett Microscopical Club, Series 2, Volume XII, 1913–1915, pp. 283–286
- Booth, A. D., The measurement of numerical aperture, Journal of the Quekett Microscopical Club, Series 4, 7, 1965, pp. 95–99
Pam provided some materials and instructions for Cheshire apertometers that can be used without any special or expensive equipment.
Types of Cheshire apertometers
(Click for full-size image, then right-click, save and print)
If this scan of the three types of Cheshire apertometers is printed to fill an A4 sheet, the dimensions should be right. The rectangular one with the 0.5 marking is 15.3 cm long and 6 cm wide, the Baker version is 11.75 cm wide and 6.6 cm wide and the other one is 14 × 13.7 cm.
Apertometer table (Click for full-size image)
The apertometer table above is from the Journal article where the dimensions of the circles are calculated. Note that if a 1 inch focus block is used, the radius of the circle is in inches (0.09 inch). So if a 2.5 cm block is used the radius of the first circle would be 0.09 × 2.5 cm.
Stephen Parker showed two versions of a Cassegrain Bireflecting Dark Ground Illuminator that was designed by E. M. Nelson and sold by Watson & Sons. It can be used with oil-immersion objectives with NA up to 1.37 (with no need for a funnel stop or iris diaphragm to reduce the NA), and also with dry objectives with NA 0.65 and above, but its vertical position has to be exactly right.
Nelson Cassegrain Bireflecting Dark Ground Illuminators
Nelson Cassegrain Bireflecting Dark Ground Illuminators and patch stops
Diagram of Nelson Cassegrain Bireflecting Dark Ground Illuminator
Table of patch stop sizes for Nelson Cassegrain Bireflecting Dark Ground Illuminator
Advertisement for Watson’s Cassegrain Dark Ground Illuminator
Cassegrain Dark Ground Illuminator (Watson 1935 catalogue)
(Click image to see full size version)
Stephen does not have a camera that suits the microscope that the condenser fits, but he still managed to show the dots on a diatom (Surirella gemma):
Dots on Surirella gemma, resolved using Cassegrain condenser
Stephen also showed an apparatus that was supposed to be an apertometer, and asked for suggestions on how to use it.
Apertometer
Chris Thomas also showed some interesting objectives and condensers.
Gordon Brown showed some interesting and unusual objectives and condensers, including a Beck focusing dark-ground condenser and a Watson zoom (⅔ to 1½ inches) objective that is not in any of Lisa and Nigel Ashby’s Watson catalogues.
W. Watson & Sons zoom (⅔ to 1½ inches) objective
Alan Wood showed 3 of his Olympus objectives from the BH-2 era and explained how their numerical aperture affects their use.
Olympus long-barrel objectives
The SPlan 100 dry objective (1-LB592) must be used dry (without oil) and its NA is 0.95, about the maximum possible for a dry objective. The high NA makes it very sensitive to coverslip thickness, so it has a collar to adjust for thicknesses between 0.14 and 0.20 mm. For best results, the collar needs to be adjusted while viewing the specimen. Its focal length is 1.68 mm, and its working distance is 0.20 mm. It cost £433.00 in 1984 and US $2310.00 in 2007.
The SPlan Apo 10 objective (1-LB730) has an unusually high NA of 0.40; most 10 objectives are around 0.25. It is an excellent objective, but the high NA goes with a working distance of just 0.55 mm, making it too dangerous for use without a coverslip. Its focal length is 16.92 mm. It cost £268.00 in 1984 and US $1742.00 in 2004. For comparison, a 10× NA 0.25 achromat cost £28.50 in 1984 and US $77.00 in 2004.
The SPlan FL 2 objective (1-LB512) has a very low NA of 0.08 and a wide field of view, useful for large specimens or for quickly scanning a slide. The normal Abbe and aplanatic/achromatic condensers cannot illuminate a wide enough field of view, so a flip-top or special low-power condenser is needed. Its focal length is 73.42 mm, much longer than the parfocal distance of 45 mm, and its working distance is 5.5 mm. It cost £198.00 in 1984.