STUDY OF A COMPOUND MICROSCOPE :- FORMATION OF THE IMAGE

MICORSCOOPE

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FORMATION OF THE IMAGE

It is the lenses composing the objective which initiate the magnifying process. An objective operates at a distance  from the object that is roughly equal to its focal length, and admits rays that are to form a real, an inverted, enlarge image in the upper part of the tube. At this point, there is interposed another lens, the field lens whose function is to collect the diverging rays of the primary image so that they may pass through the eye lens of the piece which magnifies the image still further. The field lens is accommodated in the lower plane of the eye piece. Rays, as they leave the eyes piece to reach the observer’s eye, are once more divergent and thus the image seen by the retina is virtual, appearing to be about 25 cm (10 inches) in front of the eyes.

Formation of The Image is three types :- 

A.  Magnification

B.  Resolution

C.  Oil Immersion Objective

A.   Magnification :- The  magnification  of the  object is  determined by multiplying the power of the Objective (X inscribed on the objective or eye piece denotes the Unit of the Magnification).For example, if the eye piece magnification is 10x and that of objective lens is 100x, then a given object gets magnified by 1,000 times.

 B. Resolution :- The  limit of  useful  magnification is  set by the  resolving  power  of the  lens i.e. its ability to reveal closely the adjacent structural details as separate and distinct. It is this  power which determines the amount of structural details that can be observed.

 C. Oil Immersion Objective:- Oil immersion objective is most frequently used in Haematology

                                                      because of its Greater Magnification and Resolution.

It has been seen that any factor which reduces the amount of rays falling on the objective, impairs the quality of the image. In case of oil immersion objective, the space between the object and the objective lens is filled with some transparent  fluid (such as liquid paraffin, glycerin or transparent oil) with the same refractive index as glass (=1.515) then the rays of light do not undergo refraction and pass directly into the objective. The most commonly used fluid is the cedar wood oil

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STUDY OF A COMPOUND MICROSCOPE

STUDY OF A COMPOUND MICROSCOPE

Microscope Picture

Microscope: - The microscope was invented by Antony van Leeuwen-Hoeck. It is an instrument                  by which very small things can be seen which are not seen by the Naked Eye.

                         A compound microscope essentially consists of the following Parts.

Microscope With Parts Name

Parts of Compound Microscope

1.       The Stand :- It comprises a “Heavy Foot”, often shapes like a “Horse Shoe”,   to provide stability;

And the Limb which bears the optical System. The limb is attached to the foot by a Hinged Joint so that the Microscope may be set at a Comfortable Angle for the observer.  

2.       The Optical System :- It is mounted on the tube which is usually in Two Parts.

                                                                     i.            An External Tube which bears at its lower end a revolving Nose Piece in which inter-changeable three objective lenses of various magnifications are fitted:

a.)    Low Power:- 10x

a.)    High Power:- 40x

b.)    Oil immersion:- 100x

Microscope Lenses 10x , 40x , and 100x oil immersion

                                                                   ii.            An Inner Draw Tube that carries the eye piece (Magnification 10x) at its  upper end

10x Wide Field Eye Piece (Magnification 10x)

NOTE:- The objective lenses can be identified by magnification imprinted on them. In addition , oil – immersion lens has a black rim around its lower end. It has been become customary for the objective lenses to be marked with their magnifying power rather than their focal length. The approximate equivalents are as follows: Focal length (mm)                                                          Magnification             2                                                                                    100x             4                                                                                     40x             16                                                                                   10x             40                                                                                    4x Objective lenses – low, high and oil – immersion lens

3.       The Body: - The   whole  assembly   is  held  in  position   by  the   body   which   consists  of   Two                       Mechanisms:- The Coarse and Fine Adjustments. With the help of these adjustments   the height of the tube can be adjusted in such a way so that the objective lens can be positioned at its optimal working distance (i.e. its focal length) from the object to be examined. The fine adjustment is usually graduated in 1/50ths and each division corresponds to a movement of 0.002 mm of the tube. It is used for accurate focusing.

Coarse and Fine Adjustment of Microscope            

4.       The Stage: - It is a platform that accommodates a glass slide on which the object to be examined

        is mounted. It is attached to the limb immediately below the level of the objective lens and has an aperture in the centre to permit light to reach the object.                   A Mechanical Stage is fitted to the fixed stage so that the object may be moved from side to side and before backwards.

 Mechanical Stage of Microscope                          

5.       The Sub-Stage: - It lies  beneath  the  stage  which can be lowered or raised by means of a screw.

The Stage is fitted with a Condenser and Iris Diaphragm.

(i)      The Condenser consists of two lenses, it serves to condense the rays of light and to focus them on the object. The height of condenser and, therefore, the focus of light can be varied.

(ii)    Immediately below the condenser is the Iris Diaphragm that can control the amount of light reaching the object.

Sub - Stage (Condenser with Iris-Diaphragm)

6.       The Mirror: - Fitted to  the  tail  piece  below  the  condenser is a hinged double reflecting mirror

which is plane on one side and concave on the other. The plane side of the mirror is used when the condenser is used. The mirror can be tilted and rotated in all directions.

     Digital Mirror and Simple (Sunlight) Mirror

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