"Has anyone got any dirty water?". It was Friday, late afternoon, and the anguished cry of an addict signified that we were back at Malham, and ready to get down to action.

Each August Bank Holiday weekend the Malham Tarn Field Centre is taken over by around 35 microscopists and friends, for four days of camaraderie and study. Many are regulars who have been coming since the enterprise was started 11 years ago, but each year a few faces new to Malham are made welcome.

After supper on the Friday, there is always a presentation by someone whose arm has been twisted to say a few words about something that interests them or about a curio they have come across. This year, Barry Ellam was volunteered at very short notice to give a discourse on "Exhibiting at microscopy shows". Well, that was the subject on the card, but Barry was soon off reminiscing on his life in further education and his friendship with Eric Marson. Stories of illicit crayfish gathering and pond dipping, not to mention the odd corpse on the road all contributed to an hilarious impromptu discourse.

As usual there was a wide range of studies being undertaken.. These included molluscs, mosses and the life they contain, insects, diatoms and desmids. Levels of "Research" varied from my own "that looks pretty" level to the detailed identification of species and their abundance. In the latter category, Adrian Norris identified 66 species of slugs and snails during the weekend, of which four are new to Malham. Of these, two were slugs, Deroceras panomitanum, found in Malham village, and Tandonia budapestensis from the grounds of Tarn House. The new snails were Aplexa hypnorum, found in the fen, and Gyraulus laevis from the Tarn itself. Now 87 different species of mollusc have been identified in the general area of Malham. Adrian hopes to write a paper on the Mollusca of Malham Tarn during 2005.

Another "rare" sighting was a sizeable Cladophara ball, found on the Tarn edge, and brought in and dissected.

This year, the predominant species clogging up the Tarn (and a few nets!) was Gleotrichia, last seen in any quantity four years ago, but once more giving the water in the boathouses a green sheen. Sadly, the hydra that were so abundant in 2003 were totally absent.

A visit to Malham is never complete without a trip down to the "utricularia pool", the main source of "dirty water". This pool, off the board walk, can be relied upon to yield diatoms and desmids, of interest to both beginners and the experts. This year was no exception, except perhaps that it was even richer than normal. The first sample of this, circulated to the addicts on Friday afternoon, was soon eliciting gasps of delight. The water was particularly rich in desmids and a good number of stentors were also identified. In 1999 Allan Pentecost published a paper on freshwater algae and free-living protozoa within 5 km of Malham Tarn Field Centre. This identified 134 species of desmid. Bill Ells was able to add three more desmids that were not previously on any list, when he visited Malham in 2002. Although he was unable to attend the 2004 weekend, Bill was able to identify three further additions to the list, from a sample sent to him by Dennis Fullwood. These were Closterium costatum, Closterium littorale and Pleurotaenim rectum.

On the subject of the boardwalk, it was good to see that a section of the walk has been upgraded and completed to form a circular "walk" suitable for wheel chair users.

As well as molluscs and desmids there were three other "species" never before seen at a Malham Tarn microscopists weekend. These were the Canon 10D and Nikon D70 digital single lens reflex cameras, and a superb Leica stereo microscope. As the owner of one of the 10Ds I can say how much I prefer using this on a microscope to the Coolpix that I previously used. The look and feel is virtually identical to a conventional SLR, controls and buttons are designed for normal-sized hands, and there is the minimum amount of glass between the sensor and the specimen. The Coolpix was still in use, of course, and despite a certain amount of light-hearted mockery on my part some extremely good photos were being taken with the digital compact. For anyone thinking of investing in a digital SLR for microscopy, there are a few points that need to be borne in mind. The first is that, unless you buy the very expensive Canon 1Ds, the sensor occupies a smaller area than conventional 35 mm film, giving an apparently enlarged image. Thus, lower magnification objectives are required to fill the frame, compared with a conventional camera, for a given specimen. Another factor to consider is whether you require automatic exposure metering when the camera is on the microscope. The Canon provides this, but the Nikon does not. Nikon fanciers should not be too down-hearted, however. The ability to replay the image straight away provides a valuable check on exposure before the specimen is removed. In any event, exposure compensation is almost always required, at least with the subjects that I photograph. Finally, the SLRs do not offer a moving picture facility, so if you want to film or display pictures of a hydra as is crawls around on a microscope stage, you will need to retain your Coolpix.

As for the Leica stereo microscope, this was the subject of more lust than has ever been exhibited at an adolescents' disco. Although its maximum magnification was around x60, the clarity of the image enabled as much detail to be seen on the images as could be seen on higher power compound microscopes. Zoom control of magnification, fibre-optic illumination from a separate light source and incident, transmitted and dark field illumination, not to mention an apochromatic lens, all heightened the desire!

Coming back to earth, thanks are due to Michael Brian for organising the event. The dates for 2005 are August 26th to 29th.

Jeremy Poole - reproduced from the Bulletin of the Queckett Microscopical Club