equipment

There are a few things to look for when choosing a microscope for bacteria. It seems to be widely held that Darkfield microscopes are the best for looking at bacteria. My impression is that this is probably correct as bacteria stretches the capabilities of light microscopy to its limit. I have used an alternative technique using a Brightfield microscope which might not be as effective. These are the specifications I have used:

Objective Lens
A 100x 'oil' objective lens is required. The objective lens is nearest to the slide. An 'oil' lens uses a drop of oil on the slide and the objective lens actually dips into the oil. This is not as messy or complicated as it might sound. A little bottle of special oil lasts for ages and only costs a few pounds. The quality of the objective lens is critical to how much detail a microscope can resolve.

Mechanical Stage
When a slide is placed on a mechanical stage it can be moved accurately by very tiny amounts by means of two control wheels. This is useful for high magnifications. Mechanical stages can be bought as an accessory for some microscopes that do not have them built-in.

Fine Focus
Microscopes with this facility have two focussing controls, one for course and one for fine adjustments. For high magnifications a 'fine focus' control is very useful and this must be built into the microscope.

Illumination
For high magnifications a strong light source is needed. I use a 20 watt halogen lamp adapted from a table lamp. Microscopes often have a built-in 20w light source but these are often poorly collimated and might need extra focussing. A simple convex lens (magnifying glass) on top of the light-source will often do the job of focussing the light right into the centre of the condenser diaphragm.

Condenser
A condenser is positioned below the stage that the slide sits on. It focusses the light into a bright beam. This should be focussable itself by being able to move up and down. It should also have a variable aperture/diaphragm (hole for the light to pass through). Condensers whose apperture cannot be adjusted small enough (many only close down to 1.5 - 2mm) can have a custom apperture placed in the filter holder. This is a simple disc with a hole in the centre with a piece of aluminium foil taped over it. A fine needle is used to pierce a hole of around .7mm in the middle.

Microscope specification
summary:

Objective Lens 100x oil

Eyepieces 10x and 20x or 15x

Mechanical Stage

Fine focus

Bright light source (20w min)

Condenser, focussable, variable aperture.

Camera
I use a Fujifilm F10 which is a compact digital camera with 6.2 megapixels, 3x zoom, 2.5" display (which is a good size for viewing), and light sensitivity up to ISO 1600. A highly light sensitive CCD gives more contrast at all settings and it is contrast that is needed to discern bacteria. The F10 camera includes a mains adapter for saving the battery. The 16mb picture card that came with the camera is too small for videos; a 500Mb picture card will record 7 minutes of 640x480 video at 30 frames per second.

The camera is very light and I find I can easily hold it steady against the eyepiece of the microscope for taking pictures or short movies. I also use a tripod when I am using the camera for examining a slide though dedicated camera adapters are available for fitting a camera to a microscope.

Lens Cleaning
Using a high contrast brightfield setup shows up every blemish on the lenses and keeping them clean enough is difficult. Always cover the eyepiece when it is not being used.

I found out the hard way that it is essential to keep the camera lens spotless. Setting up the microscope for high contrast means that every speck on the lenses shows up.

Microscopy websites have useful information on techniques for cleaning lenses using special lens tissues and other tools. I have learned the hard way and can recommend the following with confidence!

Never blow on a lens. Never spray lens cleaner directly onto a lens. Only use suitable lens tissues for cleaning a lens.

Other items I have used
Cavity slides: have a 15mm circular indentation in the centre. These are really useful for keeping a blood slide for days or weeks.

Coverslips: are very thin squares, rectangles or circles of glass that are placed over the specimen on the slide. I mostly use oblong slips 22mm x 40mm and squares 22mm x 22mm.