Lab ObjectivesAt the conclusion of the lab, the student should be able to: Show
Things you should be able to explain to someone else after this lab:
SlideshowNoteThe light microscopes used in this course are sensitive and expensive instruments that are handled by many students throughout the semester. This lab will teach you the information and skills you need to use and care for the microscopes properly. BackgroundMany organisms (bacteria) and parts of organisms (cells) that biologists study are too small to be seen with the human eye. We use microscopes to enlarge specimens for our investigation. The word microscope means “to see small” and the first primitive microscope was created in 1595. There are several types of microscopes but you will be mostly using a compound light microscope. This type of microscope uses visible light focused through two lenses, the ocular and the objective, to view a small specimen. Only cells that are thin enough for light to pass through will be visible with a light microscope in a two dimensional image. Another microscope that you will use in lab is a stereoscopic or a dissecting microscope. This type of microscope uses visible light view thicker, larger specimens, such as an insect, in 3D. Since you are viewing larger samples, the magnification range of the dissecting microscope is lower than the compound light microscope. Your instructor will review the parts and functions of the compound light microscopes that we will be using throughout the semester. Fill in the table on the next page to help you remember this important information. You will likely refer back to this page frequently. Here is a picture of a light microscope for you to label and take notes on.
Rules and Instructions for using the Compound Light MicroscopesYour instructor will discuss the use of the microscope with your class. The proper steps to follow for correct focusing will be reviewed. You are to follow the step-by-step sequence as your instructor indicates. Even if you are familiar with this type of microscope, you are still expected to move through the focusing review with the rest of the class.
Part 1: Total MagnificationMagnification is the ratio of the image size with the microscope to the actual size of the object. When you say that the magnification is 10, the image you see using the microscope is ten times bigger than viewing the specimen with the naked eye. Remember with a compound light microscope you are magnifying with two lenses, so to calculate the total magnification you multiple the objective magnification by the ocular magnification. View the microscope and use the chart below to calculate total magnification for each lens:
Part 2: Inversion PhenomenonObtain a letter “e” slide available in the classroom. View the slide with your eyes, and then place it onto the microscope. Use the focusing sequence to view the slide under low power. Procedure
State the inversion phenomenon in your own words. Part 3: Field of ViewThe field of view is the amount of the specimen you see when you look through the objectives. The field of view decreases at higher magnifications. ProcedurePlace a blue plastic ruler across the stage aperture so that the edge of the ruler is visible as a vertical line along the field diameter. Estimate the field size in millimeters for each of the objective lenses. Scan __________ Low (10X) __________ High (40X) __________ Part 4: Depth of FocusThe depth of focus is the thickness of the specimen that remains in focus at a given magnification. Depth of focus decreases at higher magnifications. ProcedureObtain a colored threads slide, and view it under scan or low power. Then, determine which color thread is:
Hint: using the instructions for focusing, focus on the area where the three threads cross. Use the fine focus to discern the order of the threads. Part 5: Making a Wet MountThroughout the semester, you will be expected to successfully make a number of simple slide preparations called “wet mounts.” The specimen to be observed is placed on a clean slide, a drop or two of water added, and a cover slip carefully placed over the water and specimen. Your instructor will demonstrate this technique. Make a wet mount of pond water following the procedure below:
Hopefully you will see some live organisms in your pond water. If you view green material, it is probably some sort of plant. Look to see if there is anything moving. In the space below, describe what you are observe under the microscope and draw a simple picture. Part 6: Stereoscopic Dissecting MicroscopeYour instructor will demonstrate proper use of the dissecting scope. These microscopes generally give a lower magnification than the compound microscope you are using. ProcedureView the specimens available at the lab table using the dissecting microscope. Notice that the microscope has two light sources, one from the base and one from above. The specimens are not always mounted on a slide. Also note that when you view objects under the dissecting microscope they are three-dimensional. Lab Questions
Cleaning and Caring for the Microscope
What is the total magnification of this microscope using the high power objective lens?The high-powered objective lens (also called “high dry” lens) is ideal for observing fine details within a specimen sample. The total magnification of a high-power objective lens combined with a 10x eyepiece is equal to 400x magnification, giving you a very detailed picture of the specimen in your slide.
Which part of a compound light microscope should a student adjust to allow more light?Look through the eyepiece (1) and move the focus knob until the image comes into focus. Adjust the condenser (7) and light intensity for the greatest amount of light. Move the microscope slide around until the sample is in the centre of the field of view (what you see).
How should a student adjust the microscope in order to view a greater portion of the root tip?how should a student adjust the microscope in order to view a greater portion of the root tip? switch to a lower power and decrease the size of the opening of the diaphragm.
Which of the following statements best explains the variation in the beaks of these four species?Which of the following statements best explains the variation in the beaks of these four species? Over time, an abundance of seeds for food led to increased differences between the species.
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