Download MBBS (Bachelor of Medicine and Bachelor of Surgery) Practicals Latest Physiology Foundation Module Microscope
Learning objective
By the end of this lecture, the student of first
year MBBS should be able to:
1. Perform Focusing the slide under microscope
for oil immersion lens, independently.
Microscope
It is an optical instrument by the
help of which one can magnify
and resolve microscopic
structures.
It is an instrument which is meant
to see the minute objects which
we are unable to see with naked
eye.
Microscope is an instrument of magnification and
resolution
Magnification is the ability to make small objects seem
larger, such as making a microscopic organism visible.
Resolution is the ability to distinguish two objects from
each other.
Components of microscope
1.Frame work which consist of Arm and base.
2.Mechanical stage.
3.Lens system in this system
(a) Nose piece which is movable and it holds the
objective lenses.
(b)Head and body tube it holds the objective and
ocular lens.
4.Coarse adjusting knob.
5.Fine adjusting knob.
6.Light source
Frame work of microscope
The arm and base together make the frame
work of microscope. The rest of the structures
are attached to the base and arm.
Base: is a disc shaped or U shaped support
upon which the microscope rest.
Arm: The arm is the curved ,upright structure
which is attached to the base.
Stage of compound
microscope
The stage is a horizontal plat form and
contain a centrally located opening
through which light can be reflected by the
light source.
It has two clips for holding a slide.
The stage has two parts
One where slide is placed.
Other part is the mechanical stage which
has measuring scale and knobs to move the
slide and to record the previous position of
the slide.
LENS SYSTEM
It consist of
Condenser.
Eye piece(Occular
lens).
Objective lens.
Condenser
Condenser is made up of two convex lenses. It condenses the light rays on the
object.
A movable lenses is located below the stage can be elevated or lowered using
the control knob.
Iris Diaphragm is attached to it, similar to shutter of a camera both condenser and
iris diaphragm regulates the amount of light from the light source.
When the condenser is elevated nearest the stage, the greater amount of light is
directed towards the specimen when lowered less light falls on the specimen.
Heavily stained slides for example blood smears require more light for observation
then unstained specimens.
The use of Oil immersion objective usually necessitates placing the condenser as
close to specimen as possible and fully opening the diaphragm in order to
provide the maximum amount of light.
Eye piece or Ocular lens
The eye piece is the upper
most part of the optical
system.
The magnification of the Eye
piece is (10X).
The real image produced by
the objective lens is further
magnified 10 times by the
ocular lens.
Objective lenses
There are three types of Objective lenses located on nose piece
Low power objective(10X)---Yellow
High power objective(40X)----Blue
Oil Immersion objective(100X)---white
Low power objective is used to initially focus the microscope for all type of
specimens.
It allows the rapid location of the specimen.
It is the only objective that will not touch the slide when microscope is
completely racked down.
Adjustment of Intensity of
Illumination
General principle
When we use low power(10X) Objective, a large area of field is visualized, so
needs less illumination. When oil immersion lens(100X)is used, a very small area of
field visualized, requiring highest illumination so that sufficient light reaches up to
the eyepiece.
Means to increase the intensity of illumination
1.Aperture size of diaphragm
Small size--less illumination.
Big size---more illumination.
2.Position of condenser
Lower position--minimum illumination.
Highest position--maximum illumination.
Objective lenses
Low power objective lens has light aperture larger in diameter.
While working on low power objective lens condenser is adjusted
in lower position away from the stage Iris diaphragm is slightly
open.
High power objective lens diameter of light aperture is not too
small nor so large. While using condenser should be adjusted in
the middle nor to high nor to low and iris diaphragm is moderately
opened.
Oil immersion objective lens diameter of light aperture is pin
pointed small. While using condenser should be adjusted high
near to stage and iris diaphragm is fully opened.
Use of Cedar wood oil or Liquid
paraffin in oil immersion
objective
Refractive index of cedar wood oil or liquid
paraffin is equal to that of glass, so it prevents
the divergence of light rays and the image
formed will be more clear when it is used with
oil immersion lens.
Working distance
The distance between specimen and
the objective is called working
distance.
Decrease working distance is inversely
proportional to magnification.
Low power objective lens(10X)=5mm.
High power objective lens(40X)=0.35-
0.45mm
Oil immersion lens(100X)=0.1-0.3mm.
Total magnification
The ocular lens and objective lens contribute to
the total magnification of the specimen
Ocular lens x objective lens=Total magnification.
Low power objective10x10=100x.
High power objective10x40=400x
Oil immersion objective 10x100=1000x.
Procedure for focusing a
specimen on microscope.
ALWAYS receive and handover microscope in Low
power.
Turn on the light and rotate the nose piece to low
power objective.
While looking at the slide increase the working distance
as far as possible.
Place a prepared slide on the stage, secure it with the
slide clamp and centre it using the knobs.
While still observing from the side decrease the working
distance as far as possible.
Place your eyes over the oculars and begin
initial focusing of the specimen by slowly
increasing the working distance using the coarse
adjustment knobs.
After the specimen is in focus, adjust the amount
of light using the condenser and iris diaphragm.
Using the fine adjustment knob, focus to the
sharpest possible image.
With parfocality when the a sharp image is obtained with
low power objective you will view the specimen with high
power or oil immersion objective with little or no additional
focusing.
Parfocality is the property of microscope that once the
object is focused sharply and working distance is adjusted
if the lens is changed focus remains the same only fine
adjustment is needed.
How to use oil immersion lens
Put a drop of cedar
wood oil after placing
the slide on the stage
just over the area
which has to be
focused.
Dip the oil immersion lens very carefully
with the help of coarse adjustment screw
by constantly looking from the side of
microscope so that it will just dip in the oil
without touching the slide.
Do fine focusing with the help of fine
adjustment screw by raising the objective
till its view becomes very clear.
Care of microscope
Always during transport of microscope hold it in upright
position with one hand underneath the base and other hold
the arm.
Keep the microscope at least 6 inch away from the edge of
the table.
Donot remove any part of the microscope yourself.
Never touch the lens with fingers.
Do not allow the liquids for example alcohol, water to come in
contact with microscope.
Do not use dry cotton to clean the lens.Xylene with soft cloth
should be used for this purpose.
Do not use spirit to clean lenses since it may dissolve the fixing
material of the lens.
For wet specimen use cover slip.
Always increase the distance between stage and slide
maximum while changing the slide.
Whenever microscope is not in use switch off the light.
After the work is finished switch the low power in place
again.
Task
Draw the diagram in your practical copy
Write the steps of procedure
Answer the questions given in copy
Get the practical checked by the teacher.
This post was last modified on 30 November 2021