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Recommendations....What and where to use and how much?....Telescopes and binoculars
Choosing optical instruments
Magnifiers, nature viewers and microscopes - all of these may find several uses in Topics loosely encompassed under Environmental Studies. What then are the central ideas to be borne in mind when buying any such items and in deciding what is best to use when and where? The basic principles are not at all difficult to understand. Succinct and accurate explanation - without lots of maths and trigowhatsits - that's not quite so easy!
| Thinking small That some things are just too small to see with the unaided eye is familiar from everday experience. For me it gets more familiar with each year that passes! The unaided human eye can easily separate lines or similar objects as close as 4 per millimetre at 250 mm or so from the eye. In ideal circumstances, it is possible to resolve (distinguish), objects spaced much closer than that. At their limits the human eye and brain are able to detect a visual angle equal to only one minute of arc - one sixtieth of a degree. It is easier to grasp this idea in terms of real objects in, say, a typical classroom 10 metres long. At one end of that room a pair of parallel lines, each about 3 mm wide and 3 mm apart would just be discernible as such to a naked eye at the other. With any closer spacing however a pair or more these lines would not be seen distinctly as such. To pick out a separation of only 3 mm from 10 m away is still good going. Wondrous things the eyes - how do they do it? |
The eyes have it
Everything depends on the structure of the eye. Essentially it is that of a lens system (cornea and lens) which forms an image of what's before it, on a mosaic of light sensitive nerve cells - the retina. These cells are tiny, closely packed and, in everyday circumstances, provide all the information necessary about any object in the field of view and provoking interest. To distinguish finer detail the image must be spread out on a bigger area of the retina until its components again become distinct. In other words, only a part of the original field of view must now cover more of the retina. |
| Taking a closer look
In the ordinary way this is done by bringing the object closer to the eye so that it takes up more of the field of view (below). As many adults are acutely aware, there is a limit to this expedient! This is because, unlike those of children, adult eyes are unable to accommodate (automatically focus 1) at distances much closer than 250 mm, ten inches, or so. This is known as the least distance of distinct vision. We need a little help from our lens. |
Getting even closer
A magnifying glass simulates an even closer look because it can artificially spread out the image on the retina. This brings us to another important idea which is that the eye and brain judge sizes of objects by their apparent distance from the observer. This is done by measuring visual angles. A lens system may increase the apparent angle. The brain then judges the object to be closer than it is. And if it's closer why then - it must be bigger! This is a key function of the lenses in many optical instruments whether magnifier, microscope, binocular or telescope. Getting a magnified image of almost any size is not difficult. Strangely, this may be the least of our problems. |
Seeing more
Let's extend our classroom example and suppose that we look through an optical instrument at two lines which are so close together that the naked eye cannot separate them. Suppose also that all that our instrument does is make the image bigger. It doesn't collect enough of the light leaving the object or transmit it faithfully to the image position and fully remake the pattern of the object. We may then still see one black blob - only now it's a bigger black blob! This is known as empty magnification. The device has failed to resolve detail which we know was there in the object.
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Lack of space precludes too much detail on specific instruments. Table 1 provides a general summary of what types of equipment are best for a range of typical applications. Part II of this article will contain a more detailed table with recommended models and suppliers etc. It will also deal with other kinds of optical instruments such as telescopes and binoculars.
| Magnifiers Unless you have loads of money (hollow laughter) spend the first slices of cash on good quality magnifiers. Note the limitations of devices where the distance between object and lens cannot be altered (e.g. Nature Viewers, Midi- and Minispectors, lenses on stands or fixed to a torch-like illuminator etc). Note - some of the newer flat magnifiers based on Fresnel type lenses may be very convenient for some applications and are well worth considering. |
Microscope look-alike Next - if you must have the caché of something that looks like a microscope - consider good quality fixed magnification devices such as Offord's x20 Scientist. This is versatile, can be used for solid objects as well as slides of sections, and resolves detail well at its limited magnification. An excellent explanatory booklet with lots of ideas for activities comes with this microscope. It has the further advantage that the images are upright. This means that when the object being looked at is moved the image appears to follow in the same direction. For young pupils this is pretty useful. |
Stereomicroscopes Got an energetic and generous PTA and a couple of hundred pounds or more to spend? Assuming there is full provision of magnifiers and other simple devices consider the purchase of a stereomicroscope. These are really two low-power microscopes in one (school models are usually x10, or x20, or both x10 and x20). Because they provide a separate image at each eyepiece they produce a three dimensional effect giving depth to the final single image. Another major advantage is that they do not produce inverted images. This means that, as in simple instruments - such as the Offord x20 Scientist, movement of the image follows that of the object. Types on a long-arm stand are more versatile since with them parts of large solid objects can be examined but, they are expensive. |
| Conventional microscopes You already have all of the above? Now might be the time to consider the purchase of a conventional, compound microscope at prices upward of about a hundred pounds or so. Their use in Primary should be largely reserved for P7. Although small solid specimens may be examined with lighting from above (top-light) this may mean images are seen in silhouette only. Specimens for this type of instrument are usually thin, transparent sections lit from below (transmitted light). This brings problems even at secondary level and may lead to severe difficulties for younger pupils. The snag is that two-dimensional images have somehow to be tied back to a three dimensional whole. This is not at all easy especially if we also expect the children to relate structure to function. A further difficulty is the business of inverted images. The compound microscope usually inverts both up-for-down and side-to-side. So, if the pupil moves the slide to the right the image moves off to the left. If he or she moves the slide away from him or her the image will shift toward them. All of which means that any use of a proper, compound microscope in primary courses must be carefully considered and selective. A final point concerns the issue raised earlier on the primacy of the resolution of necessary detail rather than of mere magnification. Whatever else you do, do not be taken in by catalogue descriptions or adverts for "Powerful microscope - quality optics magnifies up to 600 times . . . £32". At that price the untrue bit is undoubtedly the word "quality". Such an instrument will suffer from a host of optical faults and will grossly lack resolution. It will be little or no better than a toy. Don't buy it. |
Magnifiers - Before you buy any magnifier first obtain Offord's Understanding Magnifiers kit and then - never mind the kids - try out the worksheets and the practical exercises for yourself. Magic! This will provide a practical understanding of the key points made in this article. You should then be in a much better position to judge for yourself the quality, and value for money, of any other magnifiers you may be thinking of buying.
Microscopes - This may be beginning to read like an advertising feature for Offord. But, remember, our advice is independent. If you don't already have one then, before you buy any additional equipment in this line, first buy an Offord x20 Scientist then - as for magnifiers - do the exercises in the Offord booklet A World to Discover with a Microscope.
This may well provide you with a lot of good ideas on possible project and other work for the pupils. More importantly however it will also give you a practical understanding of what is meant by resolution and good general optical quality. These really are pre-requisites to making further sound buying decisions on other optical equipment.
What and where to use and how much?
| Specimen Type | Magnification Needed | Suitable Instruments (expected cost) |
|---|---|---|
| Minibeasts (e.g. insects and other invertebrates) external appearance of plant parts | x2 to x20 | x2 to x5 Nature viewers Minispector £3-5 x2 Osmiroid Magnispector £15 x6 to x10 folding hand lens (must be used close to eye and object) £3-12 according to diam. & quality x10 or x20 stereomicroscope £90-300 |
| Detail in structure of everyday materials such as (salt, sugar etc.) as fabrics, paper, crystals | x6 to x10 x20 |
Magnifying glass/hand lens see above Simple microscope (e.g. Offord x20 Scientist, Motic MS-2) £30-60 depends on source |
| Prepared slides and thin sections gross anatomy | x 20 | Simple microscope See above |
| As above with finer detail of internal structure | x100 to x200 | Junior microscopes £90-150 upwards (P7? and S1/S2) or use 35 mm transparencies |
| Micro-organisms such as yeast or bacteria | x400 and more | Advanced microscope or use 35 mm transparencies £150 or more so avoid at this level |
Watch out for Primary Newsletter 3 wherein we shall try to explain all that gobbledeygook about ten by forties and seven by thirties.
