Study of Compound Microscope

 

Aim: To study the structural parts and operation if Compound microscope.

Objectives: 

•  Name and identify the major parts of the microscope.
•  State the function of each microscope part.
•  Demonstrate proper use of the microscope.
• Demonstrate proper care and storage of the microscope.

THEORY :

The compound microscope is an optical instrument that is used to obtain a magnified and highly resolved  image of minute objects the details of which are not visible to the unaided eye. It makes use of two sets of lens to obtain magnification.

1. The first set known as the objective lies near the object  and is used to produces a real  and magnified image of the object  at a distance less than the distance of distinct  vision.
2. The second set known as the eye piece lies  near the eye and is  used to obtain a magnified virtual image of the  real image produced by the objective, at the distance of distinct vision so as to obtain an image with a good resolution.

Parts of a compound microscope.

1. The eye piece: It is the second set of lens in the compound microscope that is located near to the eye. It is housed in a cylindrical holder that can be slide into the draw tube. It uses of the real and magnified image of the object produced by the  objective at a  distance less than the distance of distinct vision at the source. It is used to obtain a virtual magnified and erect image at the distance of distinct vision. It is available with a magnifying power of 10X and 15X.
2. The draw(body) tube: It is a short cylindrical tube that houses the eye piece at its upper end and the rotating nose piece at the other. It ensures that eye piece and the objective are co-axial with each other and also at a constant fixed distance from each other. It also restricts light reflected from sources other than the object(glare) from reaching the eye and reducing the contrast of the image.
3. The rotating nose piece: It is a circular metallic disc that is fixed to the lower end of the the draw tube. The upper surface is concave whereas the lower surface is convex with another thin metallic disc attached to it coaxially. The lower disk has three openings to attach the objective lenses and can be rotated so as to bring the desired objective  lens into coaxial position with  eye piece. It allows  easy exchange of the objective  lens to alter the desired magnification.
4. The objective lens: It is the first set of lens in the compound microscope that is located near the object. It is used to produce a real  and magnified image of the object within the draw tube at a distance less than the distance of distinct vision. The objectives in use are of  three  type low power objective(10X), high power objective( 40X) and oil immersion objective (100X). Each of the  objective lens is fixed to the  lower end of  a short hollow cylindrical holder whose upper end can be  fitted  into  a socket in the revolving nose piece. The change in the  distance between  the objective and the eye  piece that is essential to obtain a clear image is achievied by fixing  the length of the  cylindrical  holder that  carries the objective lens.
5. The stage: It is rectangualar platform  located inferior to the  objective. A small circular  hole  in its centre allows a regulated amount of light to pass through  the specimen under observation and reach the eye. The upper surface is black  in color and non glossy to reduce  the glare  produced due to reflection of light from other source. Some have a mechanical device  called as the mechanical stage  with two  micrometer screws that are used to move  the slide forward  and backward or from  side to side.
6. Sub stage  condenser:  It is a  device immediately below the stage and is an assembly of a  concave lens and an iris diaphragm. It can be  moved up or down along  the  central  axis using a condenser knob. it is used to render the light coming  from  the  source parallel before it reaches the object so that the diffraction caused by the object as the light passes through it is  least, resulting in good resolution, It is  also  used to increase the intensity of light  of light illuminating the object.
7. The illumination source: It is located immediately below the sub stage condenser. A concave or plane mirror  is used to direct diffused sunlight or light from an incandescent lamp to illuminate the object through the openings of the sub stage condenser and the stage. This arrangement is done to avoid the glare that would be produced if the object reflects light its upper surface.
8. The base: It is the  lower most part of the microscope that I  s usually horse shoe shaped. It is heavy, broad and made up of metal. It has two vertical projections between  which the end of the C shaped arm of the  microscope is pivoted.
9. The curved arm or limb: It is  usually  C shaped  and made up of metal. The upper curve holds the focusing arrangment along with the optical system whereas the lower curve holds the stage and the illumination system,.

Standard operating procedure for compound microscope.

• Place the microscope on the bench in front of the light source.
• Ensure that the  stage is positioned horizontally when  the sample to be examined is a fluid.
• Adjust the minor to reflect the light  from  the source on  to the object so that  it illuminates it the object from below.

1. Adjustments for low power:

• Using the coarse adjustment knob lift the draw  tube up until it reaches the sub maximal  position.
• Rotate the rotating nose piece to bring the low power objective into coaxial position with the eye  piece ( recognized by the clicking sound that is  produced when it locks into position).
• Mount the slide containing the specimen ( mounted in a  mounting medium and  covered  with  a cover slip) on the stage and move the area to be observed into  coaxial position using  the two micrometer screws.
• Bend by  the  side of the draw tube  so as to view the slide with the eye in a plane that skims along the upper surface  of the slide. Using the coarse adjustment knob slowly bring down the draw tube until the low power objective is very near the slide.
• Look in the microscope and adjust the mirror and the iris diaphragm to obtain sufficient light inside it.
• Continue looking in it  and slowly raise the draw tube using  the coarse adjustment knob till the details  of the specimen placed on the slide is visible.
• Use the condenser knob to push down the condenser, until the maximum and even illumination is attained. Adjust the iris diaphragm so as to cut a narrow rim of light rays at the periphery.
• Refocus the specimen using the fine adjustment knob to obtain a clear image.

      2. Adjustments for high power:

• Raise the tube using the coarse adjustment.
• Inter change the low power objective with the high power using the rotating  nose piece.
• Bend by the  side of the  draw tube so as to view the slide with the eye level at it upper surface. Using  the coarse adjustment knob bring down the draw tube slowly until high power objective nearly touches the cover slip on the  slide.(about 1mm away from the cover slip).
• Look in the microscope and adjust the mirror in the iris diaphragm to obtain sufficient light inside it.
• Continue looking in and raise the draw tube  slowly using the coarse adjustment knob till the details of the specimen placed on the slide are visible. Use the fine adjustment knob to make minor adjustments that is necessary to obtain individuals best possible view.
• Do not push the high power objective any time using the coarse adjustment knob while looking through the microscope so as to avoid any permanent damage caused by the objective touching the slide.
• Open the iris diaphragm fully and raise the condenser to obtain maximum and even illumination.

      3. Adjustment for oil immersion objective:

• Raise the draw tube using the coarse adjustment knob.
• Interchange the low power objective with the oil immersion objective using the rotating nose piece.
• Raise the condenser up  and place a drop of cedar wood oil on the upper surface of the condenser lens.
• Place the slide on the stage and raise the condenser further so that the oil spreads in the capillary space between the condenser lens ands the lower surface of slide.
• Add a drop of oil  on the specimen and bring the area to be observed into coaxial position with  the lens system making use of the micrometer screws.
• Bend by the slide of the draw tube so as to view the slide with the eye level at it upper surface. Using the coarse adjustment knob  bring down the draw tube  slowly until the oil  immersion objective nearly touches  the drop of oil on the slide and spreads in the capillary space.
• Looking through  the  microscope raise up the draw tube slowly using the coarse adjustment knob until the faint details of the object is visible . Then use the fine adjustment knob to obtain the best possible view.
• Adjust the plane mirror to give maximum and even illumination  and then reduce the size of the iris diaphragm till it just affects the periphery of the field.
• Again refocus using the fine adjustment knob.

      Care of microscope

• Always keep the microscope covered when not in use as to avoid dust settling on the optical parts.
• Place it in a stable position to avoid toppling.
• Do not soil the stage with the specimen material, water, and stain or cedar wood oil.
• The oil on the condenser lens and the objective should be removed first with a soft dry cloth and then with a little xyline on it. Do not use excess of xyline as it damage the cementing of the lens.