Why must a small object seen through the ocular lens be centered in the field of view, before changing to a higher power objective lens?

Answer:

You must have the object centered before you change objectives to increase the magnification, because the field of view becomes smaller; if the object is off to the side, it may disappear when you go to higher magnification. For best viewing at high power, white light is essential.

Explanation:

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Microscope Notes

The compound microscope is a useful tool for magnifying objects up to as much as 1000 times their normal size. Using the microscope takes lots of practice. Follow the procedures below both to get the best results and to avoid damaging the equipment.

• The eyepiece, also called the ocular lens, is a low power lens.
• The objective lenses of compound microscopes are parfocal. You do not need to refocus (except for fine adjustment) when switching to a higher power if the object is in focus on a lower power.
• The field of view is widest on the lowest power objective. When you switch to a higher power, the field of view is closes in. You will see more of an object on low power.
• The depth of focus is greatest on the lowest power objective. Each time you switch to a higher power, the depth of focus is reduced. Therefore a smaller part of the specimen is in focus at higher power.
• The amount of light transmitted to your eye is greatest at the low power. When you switch to a higher power, light (and therefore resolving power, or the ability to distinguish two nearby objects as separate) is reduced. Compensate with the light control (sometimes called the iris diaphragm).

Field of View

The field of view is largest on the lowest power objective. When you switch to a higher power, the field of view closes in towards the center. You will see more of an object on low power. Therefore, it is best to find an object on low power, center it, and then switch to the next higher power and repeat.

Depth of Focus

The depth of focus is greatest on the lowest power objective. Each time you switch to a higher power, the depth of focus is reduced. Therefore a smaller part of the specimen is in focus at higher power. Again, this makes it easier to find an object on low power, and then switch to higher power after it is in focus. A common exercise to demonstrate depth of focus involves laying three different colored threads one on top of the other. As the observer focuses down, first the top thread comes into focus, then the middle one, and finally the bottom one. On higer power objectives one may go out of focus as another comes into focus.

Microscope Troubleshooting

Problem #1: The image is upside down and/or backwards.

1. Is the slide right-side up?
2. Inversion of the image is normal on some microscopes.
   • A common demonstration involves looking at the letter "e" on a slide.
   • When you move the slide left, does the image move left or right?

Problem #2: Everything is dark.

1. Is the microscope plugged in?
2. Is the power switch on?
3. Is the objective lens snapped into position?
4. Is the light control set correctly?
5. If you are on the highest power objective, did you forget immersion oil?

Problem #3: I can't find anything on low power!

1. Center the coverslip of the slide under the objective lens.
2. Focus up and down with the coarse adjustment knob.

Problem #4: When I moved to a higher power, everything disappeared!

1. Return to the previous (lower power) objective.
2. Center the object in the field of view.
3. Go to the higher power objective and use only the fine focus.

Problem #5: The image is blurry on all powers.

1. Clean the microscope's ocular lens. (Only use lens paper!)
• If you rotate the ocular and the specks move, there is dirt on the ocular lens and it should be cleaned.
2. Clean the slide. A tissue, paper towel, or cloth can be used.

Problem #6: The image is blurry only on a particular power.

1. Clean the microscope's objective lens. (Only use lens paper!)

Microscope Drawings

When drawing what you see under the microscope, follow the format shown below. It is important to include a figure label and a subject title above the image. The species name (and common name if there is one) and the magnification at which you were viewing the object should be written below the image. All relevant parts of the drawing should be labelled on the right side of the image using straight lines. Lines should not cross. Drawings should be done in pencil, while labels should be in pen or typed. Remember that total magnification is determined by multiplying the ocular x objective.

Viewing Prepared Slides

*** Don't hoard slides! You can only view one at a time, so that's all you should be holding. Return it before getting another, and if you break it, tell your instructor so that it can be properly cleaned up and replaced! ***

1. Start by rotating the objective lens to lowest power.
2. Place a slide on the stage, label side up, with the coverslip centered.
3. On LOW POWER ONLY, use the coarse focus knob to get the object into focus.
4. If you cannot see anything, move the slide slightly while viewing and focusing.
5. If nothing appears, reduce the light and repeat step 4.
6. Once in focus on low power, center the object of interest by moving the slide.
7. Rotate the objective to the medium power and adjust the fine focus only.
8. If needed, rotate the objective to the high power and adjust fine focus only.

Making a Wet Mount (Live Prep) Slide

1. Use a depression slide if possible-it will have a small indentation that holds fluid.
2. Squeeze the air out of the dropper before you put it in the sample container. (This prevents bubbles from agitating the contents of the sample bottle.)
3. Decide where to put the tip of the dropper-often the best stuff settles to the bottom!
4. While still squeezing the bulb of the dropper, insert the dropper into the sample container and partially release the pressure on the bulb. Fluid should rise up slowly. Gently remove the dropper from the sample container.
5. Increase the pressure on the dropper bulb to add a drop (or two at most) to the depression of the slide. The liquid should not overflow across the surface.
6. If you will be viewing fast moving organisms, you may wish to add a drop of thickener such as methyl cellulose or "ProtoSlo" to slow them down by making the fluid more viscous.
7. Slowly lay down the cover slip starting at a 45 degree angle with one edge touching the slide. This helps to prevent air bubbles from forming under the cover slip.
8. Remember that the microscope light is very intense and the organisms will survive longer on the slide if you turn it off when not observing.

Further Investigation

Digital microscope for Macintosh or Windows

Investigating pondwater organisms

Powers of 10 (1977 version)

Make your own microscope

Why do you have to center the specimen before moving to the next higher objective lens?

Why must you center and focus the object in the field of view under low power?

Why is it important to center the object you wish to view before moving to the next higher power?

What could happen to a small object seen through the ocular lens as you change from a low power to a high power objective lens?