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To What Part Of The Microscope Is The Arrow Pointing

To What part of the microscope is the arrow pointing

 

Objective Lens

 

Parfocal Microscope Objectives

 

“Parfocal” refers to objectives that can be changed with minimal or no refocusing.

Parfocalling compound microscope objectives is a procedure which allows you to adjust each objective lens so that it will remain in relative focus with the other objective lenses on the microscope when switching from one magnifcation to the next. If microscope objectives are not parfocalled it means that you would need to refocus the microscope each time the objective turret is changed to a different magnification.

When microscope objectives are not parfocalled it is inconvenient and can increase eye strain as well as wear and tear on the microscope. Each microscope manufacturer generally has a slightly different method for parfocalling objectives. The images below demonstrate how to parfocal the objectives on a Meiji laboratory microscope.

How to Parfocal Objectives
Remove the objectives that you wish to parfocal. You will need to remove each objective and the outer covers will need to be opened.

If you have a 4x or a 10x objective in your microscope, leave those in place. These will not have adjustable settings.

The higher power objectives will be parfocalled using the 10x objective as a base line. The other objectives will be adjusted in order moving up from the 10x microscope objective.

 

 

Microscope Objective cover image

 

Start by removing the outer cover of the objective lens. On the Meiji microscope objectives the objective cover will unscrew by turning it counter clockwise, like most threaded devices. The cover should unscrew easily and shouldn’t have to be forced.

If you are removing the covers of multiple objectives at one time make sure that you do not mix them up!

 

 

 

 

Microscope Objective image

The arrow in this image is pointing to the ring that allows you to adjust the parfocallity of the objective lens.

Begin by trying to turn the ring. If the ring moves then you may skip the next image and set of instructions. If the ring does not move it is most likely because there is a dab or drop of optical cement that is keeping it from rotating.

You may have to look closely in order to locate the drop of optical cement holding the ring in place.

 

 

Objective optical cement

You can remove the optical cement on the objective adjustment ring. If it can’t be removed with your finger nail you may want to lightly moisten a Q-tip with acetone and carefully remove the dab of cement. You may need to work the ring back and forth to make the ring spin freely.

After the ring is free re-install the objective (this time without the cover on it) on the microscope. Make sure you put the next highest objective after the 10x so the objectives ascend in order. (For example, place the 10x, then 40x, 100x, etc.)

 

 

 

microscpoe objectives in orderRotate the 10x objective into postion and focus on an object. A stage micrometer is a good item to focus on, but you can use anything with lines on it. Now move up to the next objective. While you are looking through the next microscope objective rather than using the focusing knobs to focus, rotate the adjustment ring on the objective.

The adjustment is somewhat sensitive, so you might have to move it slowly.

You will then repeat this process for the rest of your objectives. When you are done with all the objectives you should be able to quickly cycle through each one without having to re-focus your microscope.

 

 

Finally, you may want to apply a drop of optical cement to secure the objective ring that you just adjusted. This is not required, but is sometimes helpful in keeping your objectives parfocalled.

If you don’t have any optical cement just substitute clear nail polish. You can add the cement while the objectives are still in the microscope nosepiece. The photo at left shows where to apply the cement.

Once the cement has dried replace the objective covers and screw the objectives back into the nosepiece of the microscope. Parfocalling of microscope objectives should only need to be performed once, so you should be done with this task.

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The Microscope  Parts and Specifications

Historians credit the invention of the compound microscope to the Dutch spectacle maker, Zacharias Janssen, around the year 1590.   The compound microscope uses lenses and light to enlarge the image and is also called an optical or light microscope (vs./ an electron microscope)The simplest optical microscope is the magnifying glass and is good to about ten times (10X) magnification.  The compound microscope has two systems of lenses for greater magnification, 1) the ocular, or eyepiece lens that one looks into and 2) the objective lens, or the lens closest to the object.  Before purchasing or using a microscope, it is important to know the functions of each part.   

Eyepiece Lens:  the lens at the top that you look through.  They are usually 10X or 15X power. 

Tube:  Connects the eyepiece to the objective lenses

Arm:  Supports the tube and connects it to the base

Base:  The bottom of the microscope, used for support

Illuminator:  A steady light source (110 volts) used in place of a mirror.  If your microscope has a mirror, it is used to reflect light from an external light source up through the bottom of the stage.

Stage:  The flat platform where you place your slides.  Stage clips hold the slides in place.  If your microscope has a mechanical stage, you will be able to move the slide around by turning two knobs.  One moves it left and right, the other moves it up and down.

Revolving Nosepiece or Turret:  This is the part that holds two or more objective lenses and can be rotated to easily change power.

Objective Lenses:  Usually you will find 3 or 4 objective lenses on a microscope.  They almost always consist of 4X, 10X, 40X and 100X powers.  When coupled with a 10X (most common) eyepiece lens, we get total magnifications of 40X (4X times 10X), 100X , 400X and 1000X.  To have good resolution at 1000X, you will need a relatively sophisticated microscope with an Abbe condenser.  The shortest lens is the lowest power, the longest one is the lens with the greatest power.  Lenses are color coded and if built to DIN standards are interchangeable between microscopes.  The high power objective lenses are retractable (i.e. 40XR).  This means that if they hit a slide, the end of the lens will push in (spring loaded) thereby protecting the lens and the slide.  All quality microscopes have achromatic, parcentered, parfocal lenses.

Rack Stop:  This is an adjustment that determines how close the objective lens can get to the slide.  It is set at the factory and keeps students from cranking the high power objective lens down into the slide and breaking things.  You would only need to adjust this if you were using very thin slides and you weren’t able to focus on the specimen at high power. (Tip: If you are using thin slides and can’t focus, rather than adjust the rack stop, place a clear glass slide under the original slide to raise it a bit higher)

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).  Microscopes with in stage condenser lenses render a sharper image than those with no lens (at 400X).  If your microscope has a maximum power of 400X, you will get the maximum benefit by using a condenser lenses rated at 0.65 NA or greater.  0.65 NA condenser lenses may be mounted in the stage and work quite well.  A big advantage to a stage mounted lens is that there is one less focusing item to deal with.  If you go to 1000X then you should have a focusable condenser lens with an N.A. of 1.25 or greater.  Most 1000X microscopes use 1.25 Abbe condenser lens systems.  The Abbe condenser lens can be moved up and down.  It is set very close to the slide at 1000X and moved further away at the lower powers.  

Diaphragm or Iris:  Many microscopes have a rotating disk under the stage.  This diaphragm has different sized holes and is used to vary the intensity and size of the cone of light that is projected upward into the slide.  There is no set rule regarding which setting to use for a particular power.   Rather, the setting is a function of the transparency of the specimen, the degree of contrast you desire and the particular objective lens in use.

How to Focus Your Microscope:  The proper way to focus a microscope is to start with the lowest power objective lens first and while looking from the side, crank the lens down as close to the specimen as possible without touching it.  Now, look through the eyepiece lens and focus upward only until the image is sharp.  If you can’t get it in focus, repeat the process again.   Once the image is sharp with the low power lens, you should be able to simply click in the next power lens and do minor adjustments with the focus knob.  If your microscope has a fine focus adjustment, turning it a bit should be all that’s necessary.   Continue with subsequent objective lenses and fine focus each time. 

What to look for when purchasing a microscope.

If you want a real microscope that provides sharp crisp images then stay away from the toy stores and the plastic instruments that claim to go up to 600X or more.  There are many high quality student grade microscopes on the market today.  They have a metal body and all glass lenses.  One of the most important considerations is to purchase your instrument from a reputable source.  Although a dealer may give you a great  price, they may not be around next year to help you with a problem.   One dealer that we can highly recommend is Microscope World.   They offer a wide variety of instruments at very competitive prices. 

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