In Praise of the Stereomicroscope.
Stereomicroscopes (Fig 2) are, I fear, often not appreciated as much as they should be. In spite of the fact that their objective lenses have relatively low numerical apertures (N.A.s) and cannot reveal very fine detail in objects being examined, they have some features that make them more suitable for much microscopical work than a compound instrument (Fig 1)
Figures 1 and 2 (Click on any of the illustrations to see a larger version)
In the following table (Fig 3) , positive features are in green and negative ones in red.
Stereo's and Compounds: Pros and Cons
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For those starting out in microscopy, and this applies particularly to young people, the stereomicroscope can be recommended as an instrument that is comparatively easy to set up and adjust; it will reveal countless wonders that are not normally visible, usually without a need for the complicated processing and mounting techniques associated with specimens to be examined with the compound microscope.
Because the stereomicroscope provides a correctly-aligned image and has a large “working distance”, the space between the specimen and objective lens(es), it is the instrument of choice for dissection and manipulative work.
The usefulness of stereomicroscopes is clearly demonstrated by the fact that they are considered to be pieces of invaluable equipment in the laboratories and workshops of such people as Forensic Scientists, Medical Technicians, Electronics Engineers, Geologists, Entomologists, Botanists and Palaeontologists.
Two Types of Stereomicroscope.
The production of a stereoscopic image with its 3-dimensional quality is achieved by providing a slightly different view of the object for each of the observer’s eyes. A number of optical systems have been devised to achieve this but, for practical purposes, there are two types in use today.
The Greenough type (Figs 4 & 6) is effectively two low power compound microscopes, one for each eye; the optical axes of these are inclined to one another at an angle of about 16˚. The “Common Main Objective” (CMO) (Figs 5 & 7) type has one large objective lens and the two different images required, one for each eye, are formed by light passing through different parts of this lens.
Figures 4 and 5
Figures 6 and 7 Ray diagrams for Greenhough (Fig 6) and CMO (Fig 7) designs.
Reproduced from "Introduction to Light Microscopy" by Bradbury and Bracegirdle - with kind permission.
The pair of Porro prisms in each light path ensures that a correctly-aligned image is observed. In some instruments, first-surface mirrors replace the prisms
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Getting Your Stereomicroscope.
It is important to have an instrument with which one feels comfortable, especially if it is to be used regularly and / or for prolonged periods. Inclined tubes carrying the eyepieces make for a more comfortable posture whilst working in a seated position.
A sharp image across a wide field of view and some means of easily making some changes of magnification are other desirable features. Most work will probably be done with magnifications up to x50; to go beyond this and retain good image quality, one would need to buy more sophisticated optical components – at considerably higher cost! The most basic magnification changing systems are those that have inter-changeable objectives and / or eyepieces.
The most sophisticated is a “zoom” system, giving a similar effect to that used in some camera lenses; this is to be found on some CMOs and is very nice but, again, one would expect to pay a high price to see good images over a wide range of magnifications.
There is much to be said for a “stepped” magnification system; this gives a number of discrete magnifications that can be set up by moving supplementary lenses into place above the objective and / or changing eyepieces. The “stepped” system can produce very nice images and offer considerable flexibility for the user at reasonable cost.
Most work with a stereomicroscope is done with the specimen lit from above (“incident” lighting). It is convenient to have a lamp fitted to the stand but one can always use free-standing lamps; the latter approach gives the chance to use different types of light source and that can be important if, for example, a cool light is needed to avoid over-heating a specimen.
An integral arrangement for sending light through a transparent (or slightly translucent) specimen (“transmitted” lighting) will add considerably to the cost of an instrument but such provision can usually be contrived with the application of a little ingenuity; the examination of living aquatic organisms is an example of work that demands such a set-up and the use of light-emitting diodes (L.E.D.s) can provide the cool light required here.
Here is a CMO stereomicroscope (Fig 8) with an attached lamp for incident lighting and a base with a mirror below the specimen for setting up transmitted light from an external source. The independent lighting unit behind the instrument has a tungsten-halogen source and the incident light is conveyed to the specimen by a pair of fibre-optic guides. Arm rests are also fitted to enable dissection and manipulative work to be carried out comfortably.
Figure 8
When acquiring any microscope, it is very helpful to try out some different types before settling upon a preferred design and some retailers provide such an opportunity. Advice from members of this Club and, indeed, other microscopical societies is freely available; microscopists tend to be enthusiasts who are happy to share their knowledge and expertise with others.
As far as I am aware, there is only one book dedicated exclusively to stereomicroscopy; it provides a full treatment of the issues referred to above and many other relevant ones.
This Quekett Microscopical Club publication is now available. Members can buy it for £5 from our Sales Officer (see Shop section of this web-site) and at Club meetings. It can also be obtained from Savona Books - see Publications page
I wish to thank John Garrett and Brian Wilkinson for being “critical friends”, James Rider for allowing me to photograph his B. & L. Stereo. and Brian Bracegirdle for permission to use the two ray diagrams.
Lewis Woolnough.
June, 2010.