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bio lab 2 - Biology 05LA Winter Quarter 2008 Lab 2 page 1...

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Biology 05LA – Winter Quarter 2008 Lab 2 – page 1 LAB II: PRACTICAL MICROSCOPE USE AND THE CONCEPT OF IMAGE CONTRAST In this lab, we will look at some common plant and animal cells with the goals of practicing and adding to your microscope skills as well as introducing another important concept that is relevant to all forms of microscopy; contrast. Keep in mind that you are expected to use the protocols outlined in the SUMMARY OF MICROSCOPE USE PROCEDURES (presented in lab 1) for this and all subsequent microscopy exercises. Contrast . You have learned that what can be seen in the light microscope is limited by the size of the object. Another limiting factor in microscopy concerns image contrast. Contrast relates to the ability of the viewer to distinguish an object from its background. For example, it is easier to see dark spots on a white suit than it is to see dark spots on a dark suit. Problems of inadequate contrast are common to all forms of microscopy. In Biology 5 we will use two methods to increase contrast: partial closure of the condenser diaphragm and differential staining . 1. Closure of the condenser diaphragm . For each of the specimens that will be provided, your first observation will be of unstained tissue. As you will discover, it is difficult to see much specimen detail when the condenser diaphragm is wide open. However, if you slowly close the diaphragm while observing the specimen, you should see that more detail becomes visible as the diaphragm is closed. This improvement is realized only up to a certain point, after which contrast is not improved and resolving power is lost. Examine the effect of manipulating the condenser diaphragm for each of the available tissues and then make a sketch of the best images in your lab notebook. NOTE: The condenser itself must be properly focused to optimize the use of the condenser diaphragm to increase contrast. 2. Differential staining . Objects seen in the compound microscope are discerned as a result of their interaction with light. Biological material frequently absorbs only a small amount of light and thus has poor contrast.
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