All the microscopes in the lab are parfocals. That means that if the slide is in focus under one objective, it will stay largely in focus if the objective is changed. In practical terms, this means you should usually only need to use the coarse focus knob once per slide. You get the slide in focus under the lowest-power objective (where focusing is easiest), then, from that point onward, only make minor adjustments with the fine focus knobs even if you change
objectives. When you first get a new slide, you can usually determine the location of the specimen by looking at the slide while it is still in your hand. The specimen is usually a patch of color somewhere near the center of the coverslip. After you clip your slide securely onto the stage with the stage clips, use the stage control knobs to move the patch of color until it is directly over the hole in the center of the stage where the light comes through. Now when you look through the
eyepieces using the lowest objective (always start with the lowest objective) you should be able to find the specimen and get it quickly in focus. Occasionally the eyepiece or objective lenses will have specks of dirt or dust on them, making it difficult to focus on the specimen. To clean lenses, always use lens paper supplied by the lab instructor or a pure cotton swab. Do not use any other type of cloth or paper as they might scratch the lens. KimWipes are NOT lens paper, NEVER use
KimWipes on glass lenses or slides. To remove dirt with lens paper, first roll up the lens paper and try to dry brush away the dirt in a spiraling motion that circles from the center of the lens out. If that doesn’t work, moisten the lens paper or pure cotton swab with blue lens cleaning solution (do not apply the water to the lens directly) and clean in a spiraling motion from the center of the lens out. Below is a checklist for initially setting up a microscope. Every time you get a new
slide, you should use this checklist. Lab 1 Exercise \(\PageIndex{1}\) How The Virtual Image Differs From The Real ImageThe virtual image you see when looking in your microscope is not quite the same as the real image you would see with your eye. For one thing, it is bigger. For another thing, the orientation of the image is different. The two lenses in a compound microscope reflect the original image two times, in two different planes, while magnifying it. So what you think of as the “top” of your image is really the bottom, and what you think of “right” is really left. Usually this is not an issue at the microscopic level, but it is important to understand how the microscope is rearranging your virtual image. Lab 1 Exercise \(\PageIndex{2}\)
Making Simple But Accurate Line Drawings of Magnified SpecimensYou do not have to be a great artist to make a diagram of the cells and structures you see under a microscope. You only have to be careful to draw something that is approximately the same size and shape as what you see. Follow the following guidelines:
You should always have a basic understanding of what you are looking for before looking in the microscope. Tissues and other microscopic specimens can be confusing and cluttered. If you know in general what you are looking for, and, sometimes more importantly, what you are not looking for, it will make it much easier to find what you want to draw and it will make it much easier to decide how to draw it. Just remember, what you see under the microscope may look quite different from the perfect specimens that are usually found in the figures put into textbooks and websites. Use the idealized images to track down what you are looking for, but draw the specimen as it actually is, regardless of your expectations. For instance, in most textbooks, neurons, the most common cell found in nervous tissue, are drawn to look like variations of the drawing in Figure \(\PageIndex{1}\)A. Figure \(\PageIndex{1}\): A. A typical diagram of a neuron. B. An actual neuron. (Left CC-BY-SA, Jonathan Haas, Wikimedia, Right CC-BY, W. Clay Spencer, Rebecca McWhirter, Tyne Miller, Pnina Strasbourger, Owen Thompson, LaDeana W. Hillier, Robert H. Waterston, David M. Miller III) In the typical diagram of a neuron that appears in texts and on websites, there usually is a clear nucleus, and often a nucleolus visible, too. Sometimes organelles such as mitochondria are visible (there are none in Figure \(\PageIndex{1}\)A.) The dendrites are typically short and branched. There almost always is a single, easily-identifiable axon that is longer than all the dendrites and that branches as it ends. Figure \(\PageIndex{1}\)B shows an actual neuron as viewed through a microscope. If you view enough neurons through enough different types of microscopes, you can eventually create a composite diagram that incorporates features from many specimens to present a “typical” neuron, but it is unlikely that if you view a single neuron you will see everything in Figure \(\PageIndex{1}\)A. In fact, often actual specimens look very little like their textbook counterparts. Draw what you see, not what you think you are supposed to see. Just make sure you are looking at what you are supposed to be finding (for instance, a neuron and not a piece of dirt or cell debris), and then draw it as it is. In the case of the actual neuron in Figure \(\PageIndex{1}\)B, there is no nucleus visible, there is maybe one large projection and one small projection you could call dendrites – but there aren’t many projections – and neither of the projections are branched. There is one long thin projection that is probably an axon, and it is not branched. If you draw what you see, you end up with a drawing like the one in Figure 1.14B. It does not look like a textbook neuron, but it is a reasonable representation of what is there in this case. Figure \(\PageIndex{2}\): An actual neuron. (Left CC-BY-SA, Jonathan Haas, Wikimedia, Right CC-BY, W. Clay Spencer, Rebecca McWhirter, Tyne Miller, Pnina Strasbourger, Owen Thompson, LaDeana W. Hillier, Robert H. Waterston, David M. Miller III) Most students feel they “cannot draw” and are reluctant to sketch what they are seeing under a microscope. You don’t let your lack of artistic skills stop you.
Lab 1 Exercise \(\PageIndex{3}\) 1. Get a human blood smear slide. Rotate your lowest power objective into place on your microscope. 2. Follow the checklist in Lab Exercise \(\PageIndex{1}\) until you are viewing the blood smear under your 40x objective. 3. You will see mostly red blood cells. They will probably be pinkish and they will be the circles without nuclei. Occasionally, some will appear to have blank circles in their centers, but these are not nuclei. If you search around your slide using your stage controls, you will find the rare circular cells with nuclei. These are white blood cells. There will be less than one white blood cell for every 100 red blood cells. These white blood cells will probably be light blue or grey and have purple or dark blue nuclei. Their nuclei will not always be round. 4. Find a section of your slide with two or more white blood cells among all the red blood cells. 5. In the circle below, draw four or five representative red blood cells (do not draw all the red blood cells you see) and draw all the white blood cells in your field of view. Pay careful attention to drawing the white blood cell nuclei as accurately as possible.
LICENSES AND ATTRIBUTIONSCC LICENSED CONTENT, ORIGINALA&P Labs. Authored by: Ross Whitwam. Provided by: Mississippi University for Women. Located at: http://www.muw.edu/. License: CC BY-SA: Attribution-ShareAlike Figure \(\PageIndex{1}\)B. A drawing of Figure Figure \(\PageIndex{1}\)A's neuron.. Authored by: Ross Whitwam. Provided by: Mississippi University for Women. Located at: http://www.muw.edu. License: CC BY-SA: Attribution-ShareAlike CC LICENSED CONTENT, SHARED PREVIOUSLYFigure Figure \(\PageIndex{2}\). A typical diagram of a neuron.. Authored by: Jonathan Haas. Located at: https://commons.wikimedia.org/w/inde...curid=18271454. License: CC BY-SA: Attribution-ShareAlike CC LICENSED CONTENT, SPECIFIC ATTRIBUTION Figure Figure \(\PageIndex{2}\)A. An actual neuron; Figure \(\PageIndex{1}\)B. An actual neuron. Authored by: W. Clay Spencer, Rebecca McWhirter, Tyne Miller, Pnina Strasbourger, Owen Thompson, LaDeana W. Hillier, Robert H. Waterston, David M. Miller III. Located at: http://dx.doi.org/10.1371/journal.pone.0112102. License: CC BY: Attribution When storing a microscope, you should always follow this list:
Which term means that the microscope will remain in focus?Parfocal: A microscope that is “parfocal” is one which, if it is in focus with one objective, when the objective is rotated, will remain (mostly) in focus.
What is meant by the term focus when applied to a microscope?Focus: A means of moving the specimen closer or further away from the objective lens to render a sharp image. On some microscopes, the stage moves and on others, the tube or head of the microscope moves. Rack and pinion focusing is the most popular and durable type of focusing mechanism.
What do you call part of the microscope that is used to focus the image?Condenser Lens: The purpose of the condenser lens is to focus the light onto the specimen. Condenser lenses are most useful at the highest powers (400x and above).
When focusing a specimen under the microscope which objective is used first?Focusing Specimens:
Always start with the stage as low as possible and using scanning objective (4x).
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