LEITZ WETZLAR SM-LUX TEACHING MICROSCOPE WITH KÖHLER, DARKFIELD, POLARIZER, ANALYSER AND LED CONVERSION
BACKGROUND INFORMATION
In the 1970s, Leitz (now Leica) replaced their extensive and long-lived classic range of black enamel microscopes with a new range of stands. These stands were extremely “spacious” and rigid, providing superb ergonomics and great rigidity, and were designed with a distinctive, angular and very “1970s” appearance.
The new range comprised, in order of size, complexity and cost: (1) the HM-LUX student and teaching microscope (aimed at schools and colleges); (2) the SM-LUX teaching and routine microscope (aimed at universities and pathology labs); (3) the DIALUX laboratory microscope (aimed at university laboratories); (4) the ORTHOLUX 2 large research microscope (aimed at research laboratories in academia and industry); and (5) the ORTHOPLAN largefield microscope (aimed at very well-funded research groups – huge, heavy and very expensive).
The engineering quality of the 1970s Leitz microscopes is now regarded as the pinnacle of microscope manufacturing which has never been surpassed. The SM-LUX microscopes are full-sized, heavy and robust, beautifully engineered precision instruments that now make excellent microscopes for biology students, biology undergraduates and hobbyists. Instruments of this quality are no longer economical to manufacture – they really “do not make ‘em like this anymore”.
THE STAND
Construction
The microscope is of modern monocular form, with focusing achieved by vertical motion of the stage and with LED Köhler illumination built-in to the base. This mechanical arrangement is now almost universally adopted for research-grade biological microscopes – it provides great stability while avoiding problems arising from heavy loading of the focus rack by binocular prisms/camera rigs.
Cosmetic
Cosmetic condition is excellent, the only notable blemish being an institutional “PATH” identification discretely marked on one side of the stage bracket (this is inconspicuous and barely noticeable). Overall, the microscope presents as a very smart, clean and well-maintained instrument.
Mechanical
Focus controls
Leitz’s classic single-knob focusing allows rapid and easy adjustment at any magnification. If the knob is turned in one direction only, it actuates the coarse adjustment; reversal of the sense of rotation automatically engages the fine focus adjustment for about one third of a revolution. Turning the knob beyond the fine adjustment stop re-engages coarse adjustment. One graduation interval on the knob in fine focus mode is equivalent to 2 microns.
The focusing mechanism is buttery smooth and working perfectly, with the action in coarse mode being firm, smooth and positive, and fingertip-light when in fine mode.
Monocular head
Excellent cosmetic condition, with no significant blemishes. Quick release 43mm circular dovetail. Release lever working perfectly. Note that binocular and trinocular heads need at least twice the light intensity to deliver any given amount of light to the eye, and they contain very delicate and heavy prisms. Many are not properly collimated (due to poor quality control during assembly of many new microscopes, or knocks and wear in older examples), which leads to eye strain. Re-collimation is fiddly and time-consuming (and usually best left to trained service technicians). They also need to be adjusted for the individual viewer (both diopter and inter-pupillary distance settings must be adjusted for each user). Bino- and trino- heads are therefore not suitable for children (or for shared use, e.g., when instructing children or helping them find something on a slide).
The SM-LUX microscopes were designed to be modular, and so the head can be exchanged for a bino- or trinocular head if desired (it can be dismounted from the stand in seconds by simply actuating the quick-release lever and lifting it off the arm). Binocular and trinocular heads are available on the second-hand market.
This microscope, being monocular and built like a tank, is therefore an excellent choice for use as a teaching or demonstration microscope, e.g. for introducing youngsters to microscopy as a hobby.
Integral mechanical X-Y stage
The microscope stage is fitted with the original and beautifully engineered integrated X-Y mechanical stage/specimen driver, which is in excellent working order. The co-axial right-hand low-drive controls fall conveniently to hand and are smooth and precise in operation. On each axis there is a Vernier scale that allows measurements to be made to the nearest 0.1 mm. This permits the user to record the location of points of interest on a slide for later further study. It is in excellent cosmetic condition. The stage is fitted with the most popular LHS sprung claw specimen holder for holding a 3″×1″ microscope slide or other objects. It is beautifully machined and in excellent condition, with a nice action.
Nosepiece
The quintuple revolving nosepiece/turret is accurately centred and rotates smoothly. It clicks positively and accurately into place for quick and precise selection and alignment of each of the five objectives. Fitted with ball-bearing races for extremely smooth rotation. Excellent mechanical and cosmetic condition.
Condenser
The substage condenser is permanently mounted, and vertical adjustment is by a drop-down control knob.
Abbe-type achromatic, aspheric with N.A. 0.9, aperture iris diaphragm and centering screws. The top lens can be swung out of the light path to reduce NA and increase the field of illumination when using low power objectives.
In excellent mechanical, optical and cosmetic condition, with aperture adjusting lever turning smoothly with a good positive action. Height adjustment knob, centering screws and knob for turning the top lens in and out are all working perfectly.
Illuminator base
The built-in Köhler illumination and associated electrical components all work perfectly. The field iris assembly/aperture control ring turns smoothly and with a good, damped, action. There is a filter holder well for 35mm filters. The built-in field lens assembly is centred and free of any significant dirt, dust or scratches. There is no delamination or fungus on any of the lens elements.
A polarizer filter accessory (see below) is provided and sized to fit over the wall of the lamphouse filter well, where it can then be easily rotated for simple polarizing microscopy in conjunction with the analyser (which is also included in the kit).
The base contains a built-in 6V transformer with a 5-position rotary control knob for switching off-on (position 1), and then increasing the voltage in 4 steps (positions 2-5). The original bulb (BA9S 6V 5W incandescent) has been replaced with a brighter LED equivalent, and the control know now offers two levels of brightness at positions 4 and 5 (note that the voltages supplied at positions 1-3 are lower than the LED forward voltage and so insufficient to drive the LED bulb - these positions do work with incandescent bulbs). Position 4 is ideal for most brightfield observation, and position 5 is needed only for darkfield or the higher powers (40X and above).
The LED lamp provides white light, so there is no need to worry about colour correction/daylight filters (as is required with incandescent lamps). Power consumption is negligible, and LEDs have an extremely long lifetime (thousands of hours).
The lamp holder assembly is a simple push-pull fit into the hole in the RHS of the base, after inserting a fingertip into the recess at the end of the socket for purchase. It is in excellent mechanical condition.
THE IMAGING OPTICS
Objectives and eyepieces
The microscope is fitted with a Leitz Periplan GF 10X (wide-field, fully coated) eyepiece and the following achromatic DIN (45mm parfocal length) objectives, all fully-coated, parfocal and optically compatible with the Periplan eyepiece supplied:
· 4X (Achromat, numerical aperture 0.10, for use with or without coverslips, by Amscope)
· 10X (Achromat, numerical aperture 0.25, corrected for 0.17mm coverslips, by Amscope)
· 20X (Achromat, spring-loaded, numerical aperture 0.40, corrected for 0.17mm coverslips, new and of recent Chinese manufacture)
· 40X (Achromat, spring-loaded, numerical aperture 0.65, corrected for 0.17mm coverslips, by Amscope)
· 100X (Achromat, spring-loaded, numerical aperture 1.25, for oil immersion, by Amscope).
The objectives are in excellent cosmetic and mechanical condition (the 20X is new). Their optical performance is excellent, each producing beautiful images with good contrast and resolution. The magnifications available are 40X, 100X, 200X, 400X and 1000X (note that plant parts and insects are often best viewed at 20-40X, single cell pondlife is clearly visible at 40-200X, while many bacteria can be clearly resolved at 400X).
Important note regarding tube length
Until 1976, Leitz stands were designed with a mechanical tube length of 170mm – Leitz switched to the DIN standard (160mm) in 1976. However, it is important to note that the 170mm tube length stands are compatible with standard DIN (160mm) objectives provided that the original eyepieces designed for the pre-1976 170mm tube length microscopes are used (not the later Leitz eyepieces supplied with their post-1976 microscopes). This is because pre-1976 Leitz used an optical tube length which was only negligibly longer than that used in the later 160mm mechanical tube length stands, and so pre-1976 eyepieces pick up the intermediate image produced by DIN objectives at essentially the same location in the tube. This microscope is fitted with a Leitz Periplan eyepiece of the correct pre-1976 specification.
A cautionary note for beginners on oil immersion and the 100X objective
Note that oil-immersion 100X objectives should not be used by beginners and children without supervision: they are hard to focus (they have an extremely limited depth of field), messy (the immersion oil must be cleaned up after use with lens cleaning tissues and/or xylene) and easily damaged (the working distance is a fraction of a millimetre, and so they are often crashed into the slide/sample, damaging the lens, slide and even the microscope). They also require extremely careful sample preparation and lighting. Almost all living creatures that are exciting to watch down a microscope are best viewed with 10X-40X objectives at magnifications of less than 400X, and the 20X objective is typically MUCH more useful than the oil immersion 100X.
ACCESSORIES
The microscope comes with:
1. A custom darkfield condenser insert which slides (with a simple slide in/out friction fit) into the lower opening of the condenser (darkfield stop uppermost).
2. A linear polarizer filter sized to sit snugly (and rotate smoothly) on the filter well of the lamp housing, for use with the analyser for imaging with polarized light. Mounted under optical glass in an aluminium bezel housing.
3. A linear polarizing insert (analyser) for fitting under the monocular head, where it functions as an analyser when imaging with polarized light. This is mounted polaroid film, so while not meeting professional standards (analysers for research use are very expensive), it is perfectly adequate for casual polarized light observations and yields very good, striking images of birefringent elements in a sample when used with the rotating polarizer. When mounted under the monocular head, it has no noticeable effect on imaging quality, and so can be left in place for normal brightfield imaging.
4. A pair of synthetic polymer “selenite” slides, which are copies of Victorian originals by Edmund Wheeler from my personal collection - a “blue & yellow” and a “crimson & green”. These produce spectacular colour effects by acting as waveplates/retarders when using polarized light (see below for more details).
5. The original mains power cable.
6. A copy of the instruction manual.
Details of these accessories and/or their use follows:
Darkfield
Many specimens (such as single-celled pond life) are colourless and transparent, and so cannot be easily seen when light is simply shone through them. Darkfield filters redirect the light from the condenser so that the specimen glows against a dark background, massively increasing contrast and making them much easier to see (a bit like stars in the night sky).
The darkfield filter kit includes a custom condenser insert with an appropriately sized central stop which provides darkfield with all objectives except for the 100X oil immersion. It is a simply push-pull sliding fit into the lower opening of the condenser (see photos), and should be pushed into the condenser body until it meets a stop (the condenser barrel narrows just before the aperture iris assembly, and the insert should be pushed up to butt against the step formed by this narrowing so that the central stop is positioned in close proximity to the iris aperture). Optimum darkfield requires adjustment of condenser height, centration, condenser aperture setting (typically fully open), and also of the field iris aperture. Darkfield with the 40X objective may be improved by use of the highest lamp brightness (position 5).
Polarization (POL)
When the lightwell is fitted with the linear polarizer filter and the analyser is fitted under the head (see photos), the transmitted light can be progressively extinguished by rotating the analyser relative to the polarizer (by rotating the polarizer into the “crossed polars” or “crossed nicols” position). Any birefringent components present in the sample will then appear to shine brightly, often in striking colours. For example, the ingested food contents of ciliates and amoebas will often “light up” and sparkle, allowing their progress through the organism to be easily tracked, while chitinous insect parts and mineralized elements within botanical sections often react in colourful and beautiful ways. Polarization is an underused method of generating contrast in samples of living microorganisms that is very rewarding, and the kit supplied makes it very simple to set up.
Selenites (waveplates)
The colours produced by birefringent samples when examined under polarized light can be greatly enhanced by interposing optical phase retarders between the polarizer and analyser. Modern quantitative retarders are widely used in minerology and in conjunction with petrographic microscopes, and are usually sold as different types of waveplate(e.g., quarter-wave plate, half-wave plate, full-wave plate/“sensitive-tint”/”red-tint” plate) or compensators. They are very expensive. However, retarders made out of plates of selenite (a crystalline form of gypsum) was widely used in the 19th and first half of the 20th century for non-quantitative casual use to produce beautiful colour effects when using polarized light with birefringent specimens. These so-called “selenites” were often in a mounted slide format, typically in pairs: a “blue & yellow” and “crimson & green”, referring to the colours produced under crossed and parallel polars.
I have included two synthetic “selenites” which are recreations of original Victorian slides by Edmund Wheeler (I’ve covered the slides with copies of the c. 1870s paper covers for identification and to illustrate what these really useful and interesting old bits of kit look like – I am not trying my hand at forgery!). Appropriate thicknesses of polymer films rather than gypsum have been used as waveplates to recreate the old “blue & yellow” and “crimson & green” selenite slides.
The ”selenites’ included in the kit can be used singly or in pairs (one overlaid on the other), and should be placed on the rubberized upper rim of the polarizer filter. They can then be independently rotated (along with the polarizer), to yield almost kaleidoscopic effects with all of the Newton interference colours (think of the colours seen on soap bubbles and oil slicks) appearing according to the birefringent properties of the sample.
The microscope will be very well-packaged (these microscopes must be very carefully packed to avoid jolting the focus racks), with the modules dismounted and separately wrapped where appropriate. The items will be placed in a large, sturdy cardboard box with plenty of bubble wrap and elbow room.
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