Here we are enjoying the second week of the spring season and yet again, it’s snowing. -K
Spring is easily among the most productive seasons for the nature microscopist. Freshwater diatoms are at their peak in the spring, and many insects that have overwintered are out, as are the various forms that hatch as the weather warms. If there is a body of water a convenient distance from ones base, it can be very educational to chart the rise of spring by each day doing a quick survey of the contents of a drop of water. Does the density and variety of microbes rise steadily with temperature? Does it fall or continue to rise as snow-melt dilutes the body of water with a seasonal influx? Or does it just keep snowing!
Above is an image taken of a preserved snowflake. It is not a very good preparation but illustrates a couple points that should be emphasized. Firstly a bit of information on the preparation itself. Snow was allowed to fall on a clean slip and over a likely specimen was dropped a solution of polyvinyl formal. Without the addition of a cover glass it was maintained at a freezing temperature for a few minutes as the resin formed a cast of the captured ice crystals in a gas permeable clear plastic. When brought indoors the water from the ice crystals was able to evaporate leaving a hollow shell that may be observed.
At the tip of the arrow we observe a large air bubble. As a result of the method of preparation the air bubble formed because of air trapped in the resin itself. The large size of the air bubble makes its identity obvious and more of a nuisance than a distraction. To the left of the arrow we see an additional bubble much smaller in size. Such bubbles will spoil any preparation but illustrate an important lesson concerning depth of focus and optical alignment very well.
In the above image the small air bubble presents with a distinct black outline. This black outline is too large to illustrate diffraction rings well, but that is the principal behind much of the following. The black portion is caused by stacked layers of out of focus image planes. As a result of the known shape (spherical, with some distortion) of air bubbles we can easily picture it in cross section, the optics of out microscope can not however provide a clear outline of the section because the layers above or below obscure the image.
Increasing the magnification we can easily bring the top aspect of the bubble into focus. The black outline then becomes an aid in determining the actual form of the bubble. If our lighting is truly central we are able to discern that the portion of the bubble nearest the pointer is thicker than that which is farther away, as evidenced by the more pronounced black fringe.
If our lighting is not central then we can use the bubble to determine in which direction to move our condenser to bring it into alignment. In the above example we might take the lighting as generally central, which we can tell by observing the black fringes on the larger air bubble. However, to be truly accurate we can observe that there is a very light fringe just inside one edge of the larger bubble. Adjusting the condenser a very minor amount in the direction opposite that fringe we are able to obtain more perfectly aligned illumination.
Although still not entirely axial we observe that the fringe is much less in evidence. Properly aligned illumination is vital to correct interpretation of the image. We can now form a more accurate mental image of any object we elect to observe simply by sending the fine focus just above or below the image plane we wish to observe. Axial lighting provides that any dark fringes we observe will be a product of the specimen rather than an artifact of our illuminating system.