(Page Design by Anne Bruce April 1998)
Epping Forest abounds with small ponds and dikes and is therefore an excellent place to find freshwater protozoa and algae. I use a round net with two filters of 53 micrometers and 35 micrometers, these two filters hold all the specimens that I need. It is important to make sure that the pond water is placed into a suitable container and kept fairly cool, especially on very warm summer days, where heat will very quickly kill off many species of protozoa.
Many of the micro-organisms found in ponds are very difficult to observe in any detail let alone trying to photograph them. One way to increase the contrast when using brightfield illumination is to close down the substage condenser more than is normally acceptable, but this method will introduce diffraction patterns around the object and will degrade the image. Staining is another method of introducing contrast to the subject, but once again this method has its drawbacks. Fixed cells do not tell you much about how an organism behaves and also the morphology will be altered. The exciting thing about freshwater microscopy is being able to watch the little animals swimming around in all their glory.
Various methods can be used by the amateur microscopist to make the observation of living cells more revealing and also more educational, without harming or distorting the cells in any way. Darkfield, phase contrast and Nomarski are excellent tools, but can also be very expensive to buy new. Detail within the cell body can be revealed and structures such as cilia, flagella etc. are all made visible by using the above methods. If you are serious about your hobby then it is worth investing in a darkfield condenser and possibly a phase outfit when the funds become available. The human eye or camera cannot detect phase differences, but can detect differences in amplitude and this is where the phase contrast outfit comes in handy.
|The first two photographs are of an unknown species of amoeba and were taken with phase contrast at a magnification of 250x. The pseudopodia, which tend to branch out in all directions searching for that tasty morsel are one of the main features of this type of amoeba. Detail otherwise missed can clearly be seen in the two photographs. The cytoplasm of this species is differentiated into two distinctive zones, the inner fluid part, which is called the endoplasm and the somewhat clearer outer region which is in the form of a gel.|
This photograph is of the same species but taken with a darkground condenser in place. Many crystals and inclusions within the cytoplasm are clearly visible. Direct rays from the light source are prevented from entering the front lens of the objective by a stop which is situated in the substage condenser. Bacteria, flagella and cilia benefit greatly from this type of illumination.
These last two photographs were taken with a Nomarski system which gives the illusion of a 3D effect and improves upon the resolution of what can be seen when looking at algae and protozoa, due to the optical sectioning of the specimen by the illumination. The light is first polarised and then passed through a prism which splits the light up into two beams and then rotates them so they are at right angles to each other, the light then travels through the specimen and up to the objective where the beams recombine and interfere with each other. This method is particularly expensive to buy but if the opportunity to use such a piece of equipment occurs grab it with both hands The first photograph show a food vacuole with the remains of some micro-organism still in the process of being digested, while the final shot is that of the very granulated nucleus which stands out clearly. Notice the lack of any colour variations when using Nomarski.
The types of microscopes that I use are a Leitz Orthoplan large field microscope fitted with an Orthomat fully automatic camera, which has a built in zoom optical attachment. The exposure range is almost unlimited with this camera. The lenses are either planapo or plan fluorite. A Carl Zeiss microscope is also used with very similar specifications.
Comments on the article to the author Steve Durr
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