The diameters of the fields are estimated by the observation of a clear millimeter (mm) ruler. The ruler is observed under each magnification (except oil immersion) starting first with the scanning power objective. Once the diameter is determined, then the size of specimens can be estimated. The following illustrations represents magnifications with respect to millimeters. Observe that as magnification increases the diameter of the field decreases. Starting an observation with the scanning power objective allows the observation with the greatest diameter (or total observed area). One reason to never start an observation with the high or the oil immersion objective is because their area observed is too small to easily locate the specimen. Also, notice that the fields decrease in diameter around a central point. It is important to center the specimen before moving to the next higher magnification because the observed specimen may be lost due to being in the periphery of the previous field.
The slide preparation of the "Letter e" is commonly used to emphasize the observed position of the image, the movement of the stage, and the diameters of the fields. Observe the position of the "letter e" on the stage, and then observe with scanning power and compare the observed position of the image. Notice that the observed image is upside down and backward. The inversion of the image requires that to move the specimen in a desired direction, the specimen (stage) is moved in the opposite direction.
The depth of field is the vertical (top-to-bottom) distance that is in focus. The depth of field decreases with increasing magnification. Thus, the scanning power objective has the greatest depth of field and oil immersion has the shallowest depth of field. A slide preparation of three different colored "Silk Fibers" or "Threads" is commonly used to demonstrate the depth of field. Typically the slide preparation is prepared by using three different colored threads positioned to cross one-over-the-other at the same place. Thus by focusing on each thread through the vertical plane the sequence of threads is determined as to top, middle, and bottom.
Starting an observation with the scanning power objective allows the observation with the greatest depth of field (and diameter of field). Reasons to never start an observation with the high or the oil immersion objective is because their vertical (depth of field) and horizontal (diameter of field) observed-areas are too small to easily locate the specimen.
Observation of Pond Water
Pond water (scum) contains a variety of microscopic organisms and is an excellent for observations to gain proficiency in the use of the microscope. Additionally, cultures of protozoa may be purchased or maintained for observation.
The following LINKS contain excellent videos of typical organisms found in pond water.
Nikon's "Microscopy U, A Source for Microscopy Education"
Olympus Microscopy Resource Center
This site was last updated 06/30/07