Lenses near the slide in light microscope.. Objective

Lenses near the slide in light microscope:
a- Objective***
b- Stage
c- Eye lenses
d- Arms.
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Compound microscope. It is an optical microscope that has more than one objective lens. They are especially used to examine transparent objects, or cut into sheets so thin that they are transparent. It is also used to increase or enlarge the images of objects and organisms not visible to the naked eye. The common optical microscope is made up of three systems:
- The mechanical system consists of a lever used to hold, lift and stop the instruments to be observed.
- The lighting system comprises a set of instruments, arranged in such a way that they produce the light slots.
- The optical system comprises the parts of the microscope allow an increase of the objects that are intended to be observed by means of filters called "of subsequent antigel".
It seems clear that the compound microscope was invented if the term fits in 1590 by Hans (father) and Zacarias (son) Janssen, from Middelbourg, Holland (it is not uncommon to find the reference that both were brothers); mainly by Zacarias, who, they say, combined two convergent simple lenses: one operated "objective" and the other "ocular."

Mechanical part of the microscope:
The mechanical part of the microscope includes the foot, the tube, the revolver, the handle, the stage, the carriage and the micrometric screw. These elements support the optical and lighting part; They also allow the necessary displacements for the focus of the object.
- The foot and support: It is the base on which the microscope rests and is usually Y-shaped or rectangular.
- The column or arm: also called a handle, it is a C-shaped piece, attached to the base by its bottom by means of a hinge, allowing the inclination of the tube to improve the capture of light when mirrors are used. It holds the tube in its upper portion and by the lower end it adapts to the foot.
- The tube: it has a cylindrical shape and is blackened internally to avoid light reflections. The eyepieces are placed at the upper end and the objective revolver at the lower end. The tube is attached to the top of the column by a system of zippers, which allow the tube to move through the screws.
- The macrometric screw: turning this screw, ascends or descends the microscope tube, sliding vertically thanks to a rack mechanism. These long movements allow quick focus of the preparation.
The micrometric screw: by fine adjustment with almost imperceptible movement that occurs when sliding the tube or the plate, the precise and clear approach of the preparation is achieved. It has a graduated drum in divisions of 0.001 mm, which is used to specify its movements and can measure the thickness of the objects.
- The stage: it is a flat metal piece in which the preparation or object to be observed is placed. It has a hole, in the optical axis of the tube, which allows the passage of light rays to the preparation. The stage can be fixed, in which case it remains motionless; in other cases it can be rotating; that is to say, by means of lateral screws it can be centered or produce circular movements.
- The tweezers: they are two metallic pieces that serve to hold the preparation. They are on the stage.
- Mobile carriage: it is a device that consists of two screws and is placed on the stage, which allows to slide the preparation with orthogonal movement from front to back and from right to left.
- The revolver: it is a rotating part with holes in which the objectives are screwed. When turning the revolver, the objectives pass through the axis of the tube and are placed in working position, which is noted by the noise of a pinion that fixes it.

Optical system:

The optical system is responsible for reproducing and increasing the images through the set of lenses that compose it. It is formed by the eyepiece and the objectives. The objective projects an image of the sample that the eyepiece then enlarges.

- The eyepiece: it is located in the upper part of the tube. Its name is due to the proximity of the piece with the Eye of the observer. Its function is to increase the image formed by the objective. The eyepieces are interchangeable and their magnification powers range from 5X to 20X. There are special eyepieces of powers greater than 20X and others that have a micrometric scale; The latter are intended to measure the size of the object observed.

- The objectives: they are arranged in a rotating piece called a revolver and produce the magnification of the images of objects and organisms, and, therefore, are close to the preparation being examined. The objectives currently used are of two types: dry objectives and immersion objectives.

- Dry targets are used without the need to place any substance between them and the preparation. On the external face they carry a series of indices that indicate the increase they produce, the numerical aperture and other data. Thus, for example, if an objective has this data: plan 40 / 0.65 and 160 / 0.17, it means that the objective is flatchromatic, its increase 40 and its numerical aperture 0.65, calculated for a tube length of 160 mm The number of objectives varies with the type of microscope and the intended use. The most frequently used dry lens increases are: 4X, 10X, 20X, 40X and 60X.

- The immersion objective is composed of a complicated lens system. To observe through this objective it is necessary to place a drop of cedar oil between the objective and the preparation, so that the front lens comes into contact with the cedar oil. Generally, these objectives are 100X and is distinguished by one or two circles or rings of black color that surrounds its lower end.

Lighting system:

This system aims to direct natural or artificial light in such a way that it illuminates the preparation or object to be observed in the microscope in the appropriate manner. It comprises the following elements:

- Lighting source: it is classically a tungsten incandescent lamp overloaded; in more modern versions with LEDs. In front of it is a condenser (a converging lens) and, ideally, a field diaphragm, which allows to control the diameter of the part of the preparation that is illuminated, to avoid exceeding the observation field producing parasitic lights.

- The mirror: necessary if the light source is not built into the microscope and already aligned with the optical system, as is often the case in modern microscopes. It usually has two faces: a concave and a flat one. Enjoy of movements in all directions. The concave face is preferably used with artificial lighting, and the flat, for natural (sunlight). The most modern models do not have mirrors but a Lamp that fulfills the same function as the mirror.

- Condenser: it is formed by a system of lenses, whose purpose is to concentrate the light rays on the plane of the preparation, forming a cone of light with the same angle as the field of the target. The condenser is placed under the stage and its upper lens is generally flat convex, the upper face being flat in contact with the preparation when large aperture lenses (those of greater magnification) are used; There are immersion condensers, which ask that the space between that upper lens and the preparation be filled with oil. The maximum numerical aperture of the condenser must be at least the same as that of the objective used, or it will not be possible to take advantage of all its separating power. The condenser can slide vertically on a rack system using a screw, lowering for use with low power targets.

- Diaphragm: the condenser is provided with an iris diaphragm, which regulates its aperture to adjust it to that of the objective. It can be used, irregularly, to increase the contrast, which is done by closing it more than is convenient if you want to take advantage of the resolution of the optical system.

Path of the light beam through the microscope:

The light beam from the lamp passes directly through the diaphragm to the condenser. Thanks to the lens system that the condenser has, the light is concentrated on the preparation to be observed. The beam of light penetrates the target and continues through the tube until it reaches the eyepiece, where it is captured by the eye of the observer.

Properties of the microscope:

- Separating power. Also sometimes called resolution power, it is a quality of the microscope, and is defined as the minimum distance between two nearby points that can be separated. The normal eye cannot see two points apart when its distance is less than one tenth of a millimeter. In the microscope it is limited by the wavelength of the radiation used; in the optical microscope, the maximum separating power achieved is 0.2 tenths of a micrometer (half the wavelength of blue light), and in the electron microscope, the separating power reaches up to 10 Å.

- Power of definition. It refers to the sharpness of the images obtained, especially with respect to their contours. This property depends on the quality and correction of the aberrations of the lenses used.

- Microscope magnification. In general terms, it is defined as the relationship between the apparent diameter of the image and the diameter or length of the object. This means that if the microscope increases an object by 100 diameters, the image we are seeing is 100 times linearly larger than the actual size of the object (the surface of the image will be 1002, that is 10,000 times larger). To calculate the magnification that a microscope is providing, it is enough to multiply the respective magnifications due to the objective and the eyepiece used. For example, if we are using a 45X lens and a 10X eyepiece, the magnification with which we are seeing the sample will be: 45X x 10X = 450X, which means that the image of the object is enlarged 450 times, also expressed as 450 diameters

Microscope Maintenance:

The microscope must be protected from dust, moisture and other agents that could damage it. While not in use, it should be stored in a case or cabinet, or covered with a plastic bag or glass bell. The mechanical parts should be cleaned with a soft cloth; In some cases, it can be moistened with xylol to dissolve certain grease stains, cedar oil, paraffin, etc. That they have fallen on the mentioned parts. Cleaning the optical parts requires special precautions. For this, "clean lens" paper issued by the distribution houses of laboratory equipment must be used. Never touch the eyepiece, objective and condenser lenses with your fingers; fingerprints impair visibility, and when dried it is laborious to remove them. For a good cleaning of the lenses, the "clean lens" paper can be moistened with ether and then passed through the surface as many times as necessary. The cedar oil that remains on the front lens of the immersion lens should be removed immediately after the end of the observation. For this you can pass the "clean lens" paper impregnated with a drop of xylol. To save it, it is customary to place the objective of lower magnification on the stage and lowered to the top; The capacitor must be in its lowest position, to avoid encountering any of the objectives. Store in dry places, to prevent moisture from favoring fungus formation. Certain acids and other chemicals that produce strong emanations should be kept away from the microscope.