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Choosing the Right Microscope : 15

of space, let alone get light onto the top of it. So a microscope built optically to give a very long working distance (100 mm or so) to allow the introduction of top light and permit manipulation or dissection of the bee is needed. This is the low power or stereomicroscope.

The Low Power Stereomicroscope

Stereomicroscopes are made to examine whole objects with top lighting with a long working distance. They were originally called dissection microscopes because they are ideal for this purpose. They are binocular but are very different from binocular high power microscopes. In many ways they are put together rather like binoculars in that each eyetube has its own objective so that there are two complete but independent optical systems

joined in the middle, just like binoculars. However, in this case the two systems are at a slight angle to one another so that each eye sees a slightly left- or right-biased image and the brain clicks these into a 3D image – hence stereomicroscope. This 3D image is also (unlike the high power microscopes) the right way round so that fine dissection can be carried out with excellent judgment of distances. These stereomicroscope heads can be attached to a variety of stands. The stands can have inbuilt incident and sometimes transmitted lighting which these days is often LED. Alternatively the heads can be attached to the end of ‘poles’ to hold them out above the bench area so that the hands are free to work or an artificial insemination table can be placed under the head. This type of stereomicroscope

is usually referred to as a Long Arm Stereo. They have the advantage that the hands do not have to be held up onto the base plate of the illuminated stand – which makes dissection easier. The smaller long arms also have flexible LED light units. A budget in the region of £200 is needed for one of these. Figures 3 and 4 show examples of stereomicroscopes on illuminated stands and a long-arm version is shown in Figure 5. The latter would need an independent light source to shine onto the specimen. Budget stereomicroscopes typically have a single magnification, usually x20, which is too much for bee dissection. Still within the budget range are models that give a choice between two magnifications: x10 and x30 options are ideal for bee dissection. For those with a

larger budget, stereomicroscopes

can have a zoom magnification control – usually between x10 and x40. These are excellent but may cost more – up to £500. So the check list for the beekeeper is: • starting magnification of x10

• incident light source • LED lighting – no heat • additional higher

magnification option – x30 an advantage.

Just to complicate matters,

there are now high power and stereomicroscopes with digital cameras built into the head which means that the microscope can be attached via USB directly to a computer. These can be useful in certain circumstances but they do also have restrictions and disadvantages, but that in itself is a whole separate article. ¤

August 2013 Vol 95 No 8

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