Microscope

7/21/2019 Microscope

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MICRCOSCOPE

A microscope is a magnifying instrument. The magnified image of the object (specimen) is first

produced by a lens close to the object called the objective. This collects light from the specimen

and forms the primary image. A second lens near the eye called the eyepiece enlarges the

primary image, converting it into one that can enter the pupil of the eye.

PARTS

OBJECTIVES

The objective lenses are the most important part the microscope, determining the various

magnifications possible and defining the optical quality achievable to the rest of the instrument.

Typically X10 (low power), X40 (high power) and X100 (oil immersion) objective lenses are

attached to a rotating turret. The refractive index of air is 1.0 whereas glass and cedar wood oil

is 1.5. If the refractive index of all media is same, it results in better magnification.

The magnification of the objective multiplied by that of the eyepiece gives the total

magnification of the image seen in microscopes.Examples

Objective magnification Eyepiece magnification Total magnification

010 X 100 == 100 diameters

040 X 10 = 400 diameters

100 X 10 = 1000 diameters

EYEPIECESThe eyepieces further magnify the image produced by the objective lenses, usually by a

factor of x10. The image they produce is focused at infinity, which allows the operator to

comfortably view the image as if in the distance. ‘High eyepoint’ eyepieces are useful for

spectacle wearers because they are designed to allow the full image to be viewed from several

centimetres above the eyepiece.

CONDENSER

The condenser is an important part of the illumination system. When correctly adjusted,

it focuses a uniform cone of light onto the specimen (at low magnifications a ‘swing out’ lens

above the condenser may have to be removed from the light path to ensure that the whole

field of view is illuminated). Correct adjustment of the condenser diaphragm ensures an

optimal balance of image resolution, contrast and depth of field.

DIAPHRAGM

The field diaphragm is centered and its aperture adjusted so that only the observed

region of the specimen is illuminated. This minimizes unnecessary light scatter otherwise

produced within the unobserved outer regions of the specimen.



LIGHT SOURCE

The most commonly used light source in modern laboratory microscopes is a low-

voltage tungsten/halogen bulb. This provides stable and intense illumination in the visible

spectrum. The bulb may be housed within the body of the microscope or within an external

lamp housing.

USE OF MICROSCOPE

1- Position the microscope correctly and identify the essential parts.

2- Before switching on the microscope turn the lamp brilliance control to its lowest

setting, then increase it to about three quarters of its power.

3- Bring the 10X objective into place.

4- Make sure the underside of the specimen slide and surface of the stage are completely

dry and clean.

5- Place the specimen on the stage in the slide holder.

6- Focus the specimen with the 10X objective.

7- Focus the condenser and leave it in this position for all objectives.

8- If the microscope is not fitted with a presented condenser, check the centering of the

condenser.

9- Examine the specimen with the 10X objective. Obtain the best image by:

– closing the iris about two thirds,

– adjusting the lamp brightness control to give good illumination with the

minimum of glare.

10- Use the mechanical stage to examine the specimen systematically.

11- Examine the specimen with the 40X objective.

Obtain the best image by:

– opening the iris more,

– increasing the illumination.12- Examine the specimen with the 100X objective. Move the 40X objective to the side,

place a drop of oil on the specimen and bring the 100X objective into position.

Obtain the best image by:

– opening the iris fully,

– increasing the illumination.

To prevent damage to the 100X objective lens, move the objective to one side before

removing the specimen.



Cross-sectional diagram of a typical laboratory microscope

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Microscope