About optical microscope

Open classification: equipment, optics, machinery, microscope

Optical microscope is an optical instrument that magnifies and images tiny objects that can't be distinguished by human eyes and enables people to extract tiny structural information.

As early as the first century BC, people have found that when observing tiny objects through spherical transparent objects, the images can be enlarged. Later, I gradually realized that the surface energy of spherical glass magnifies the image of an object.

In 1590, Dutch and Italian glasses manufacturers have built magnifying instruments similar to microscopes. About 16 10 years ago, Galileo of Italy and Kepler of Germany changed the distance between the objective lens and the eyepiece and obtained a reasonable optical path structure of the microscope. At that time, optical craftsmen were engaged in the manufacture, promotion and improvement of microscopes.

/kloc-In the mid-7th century, Hooke in Britain and Levin Hooke in the Netherlands made outstanding contributions to the development of microscopes. About 1665, Hooke added a coarse focusing and fine focusing mechanism, an illumination system and a workbench for carrying samples to the microscope. These components have been continuously improved and become the basic components of modern microscopes.

During the period from1673 to1677, Levin Hook made a single-component magnifying glass, nine of which have been preserved to this day. Hook and Levin Hook used their own microscopes to study the microstructure of animals and plants and made outstanding achievements.

/kloc-in the 0/9th century, the appearance of high-quality achromatic immersion objective lens greatly improved the ability of microscope to observe fine structures. 1827 Ameche first used immersion lens. 19 In the 1970s, Abbe, Germany laid the classical theoretical foundation for microscope imaging. All these have promoted the rapid development of microscope manufacturing and microscopic observation technology, and provided powerful tools for biologists and medical scientists including Koch and Pasteur to discover bacteria and microorganisms in the second half of the19th century.

With the development of the microscope itself, the microscopic observation technology is constantly innovating: 1850 has a polarized microscope; The interference microscope appears in1893; Zelnik, a Dutch physicist, created phase contrast microscopy in 1935, and won the Nobel Prize in Physics in 1953.

The classical optical microscope is only a combination of optical elements and precision mechanical elements. It uses the human eye as the receiver to observe the enlarged image. Later, a photographic device was added to the microscope, and photosensitive film was used as the receiver for recording and storage. In modern times, photoelectric elements, TV cameras and charge couplers are widely used as receivers of microscopes, and form a complete image information acquisition and processing system with microcomputers.

Curved optical lenses made of glass or other transparent materials can magnify objects, and optical microscopes use this principle to magnify tiny objects to the size that can be observed by human eyes. Modern optical microscope usually adopts two-stage magnification, which is completed by objective lens and eyepiece respectively. The observed object is located in front of the objective lens, which magnifies for the first time to form an inverted real image, and then magnifies for the second time through the eyepiece to form a virtual image, which is what the human eye sees. The total magnification of a microscope is the product of the objective magnification and the eyepiece magnification. Magnification refers to the magnification of linear dimensions, not the area ratio.

Composition structure of optical microscope

Optical microscope generally consists of objective table, spotlight lighting system, objective lens, eyepiece and focusing mechanism. The stage is used to place the observed object. The focusing knob can be used to drive the focusing mechanism to make the object table move up and down roughly and finely, and the observed object can be focused and imaged clearly. Its upper layer can move and rotate accurately in the horizontal plane, and the observed part is generally placed in the center of the field of vision.

The spotlight lighting system consists of a light source and a condenser. The function of the condenser is to concentrate more light energy on the observed part. The spectral characteristics of the illumination lamp must adapt to the working band of the microscope receiver.

The objective lens is located near the observed object, which is the lens to realize the first-stage amplification. Several objective lenses with different magnifications are simultaneously installed on the objective lens converter. By rotating the converter, the objective lens with different magnifications can enter the working optical path, and the magnification of the objective lens is usually 5 ~ 100 times.

Objective lens is an optical element that plays a decisive role in the imaging quality of microscope. Commonly used achromatic objective lens can correct the chromatic aberration of two colors of light; There is also a higher quality apochromatic objective lens, which can correct the chromatic aberration of three colors of light; The flat-field objective lens can ensure that the whole image plane of the objective lens is flat, thus improving the imaging quality of the edge of the field of view. Immersion objective lens is often used in high power objective lens, that is, liquid with refractive index of about 1.5 is filled between the lower surface of the objective lens and the upper surface of the sample, which can significantly improve the resolution of microscopic observation.

The eyepiece is a lens located near the human eye to realize the second-stage magnification, and the magnification of the mirror is usually 5 ~ 20 times. According to the size of the field of view, eyepiece can be divided into two types: ordinary eyepiece with small field of view and eyepiece with large field of view (or wide-angle eyepiece).

Both the stage and the objective lens must be able to move relative to each other along the optical axis of the objective lens in order to focus and obtain a clear image. When working with a high-power objective lens, the allowable focusing range is often less than micron, so the microscope must have an extremely precise fine focusing mechanism.

The limit of microscope magnification is the effective magnification, and the resolution of microscope refers to the minimum distance between two objects that the microscope can clearly distinguish. Resolution and magnification are two different but interrelated concepts.

When the numerical aperture of the selected objective lens is not large enough, that is, the resolution is not high enough, the microscope cannot distinguish the fine structure of the object. At this time, even if the magnification is excessively increased, only an image with a large outline but unclear details can be obtained, which is called invalid magnification. On the other hand, if the resolution meets the requirements and the magnification is insufficient, the microscope has resolution, but the image is still too small for human eyes to see clearly. Therefore, in order to give full play to the resolution of the microscope, the numerical aperture should be reasonably matched with the total magnification of the microscope.

Spotlight lighting system has a great influence on the imaging performance of microscope, but it is also a link that users easily ignore. Its function is to provide sufficient brightness and uniform illumination for the surface of the object. The beam from the condenser should be able to fill the aperture angle of the objective lens, otherwise the highest resolution that the objective lens can achieve cannot be fully utilized. Therefore, there is a variable aperture diaphragm in the condenser, which is similar to the diaphragm in the photographic objective, and the aperture of the illumination beam can be adjusted to match the aperture angle of the objective.

By changing the illumination mode, we can obtain different observation modes such as dark objects on bright background (called bright field illumination) or bright objects on dark background (called dark field illumination), so as to better discover and observe fine structures under different circumstances.

Classification of optical microscope

Optical microscopes can be classified in many ways: according to the number of eyepieces used, they can be divided into binocular and monocular microscopes; According to whether the image has stereoscopic effect.

Sensory microscope can be divided into stereoscopic vision and non-stereoscopic vision; According to observation, objects can be divided into biological microscope and metallographic microscope; According to the optical principle, it can be divided into polarization microscope, phase contrast microscope and differential interference contrast microscope. According to the type of light source, it can be divided into ordinary light, fluorescence, infrared light and laser microscope. According to the type of receiver, it can be divided into visual microscope, photographic microscope and television microscope. Commonly used microscopes include binocular stereo microscope, metallographic microscope, polarizing microscope and ultraviolet fluorescence microscope.

Binocular stereoscopic microscope provides stereoscopic images for left and right eyes by using dual-channel optical path. In essence, two single-barrel microscopes are placed side by side, and the optical axes of the two barrels form the perspective formed when people observe an object with their eyes, thus forming a stereoscopic vision image in three-dimensional space. Binocular stereomicroscope is widely used in biology and medicine, for slicing and microsurgery. It is used for the observation, assembly and inspection of micro parts and integrated circuits in industry.

Metallographic microscope is a microscope specially used to observe the metallographic structure of opaque objects such as metals and minerals. These opaque objects can't be observed under ordinary transmission light microscope, so the main difference between metallographic microscope and ordinary microscope is that the former is illuminated by reflected light, while the latter is illuminated by transmitted light. In metallographic microscope, the illumination beam irradiates the surface of the observed object from the direction of the objective lens, and returns to the objective lens for imaging after being reflected by the surface of the object. This reflective illumination method is also widely used in the inspection of integrated circuit silicon wafers.

Ultraviolet fluorescence microscope is a microscope that uses ultraviolet light to excite fluorescence to observe. Some specimens can't perceive the structural details under visible light, but after dyeing, under ultraviolet light irradiation, visible light can be emitted due to fluorescence, forming a visible image. This kind of microscope is often used in biology and medicine.

Television microscope and charge-coupled microscope are microscopes with television camera targets or charge-coupled devices as receiving elements. A TV camera target or a charge coupler is installed on the real image surface of the microscope to replace the human eye as a receiver, and the optical image is converted into an electrical signal image through these photoelectric devices, and then the size detection and particle counting are carried out. This microscope can be combined with computer, which is convenient to realize the automation of detection and information processing, and is often used in situations where a lot of complicated detection work is needed.

Scanning microscope is a kind of microscope whose imaging beam can scan relative to the surface of an object. In the scanning microscope, the highest resolution of the objective lens is ensured by narrowing the field of view. At the same time, the imaging beam is scanned in a large field of view relative to the surface of the object by optical or mechanical scanning, and the synthesized large-area image information is obtained by information processing technology. This microscope is suitable for observing large-field images that require high resolution. Coarse focusing screw: adjust the lens barrel up and down in a large range.

Fine focus screw: adjust the lens barrel up and down in a small range.

in addition

Microscope is a precise optical instrument with a history of more than 300 years. Since the appearance of the microscope, people have seen many tiny creatures and cells, which are the basic units of organisms and were invisible in the past. Now there are not only optical microscopes with a magnification of more than 1000 times, but also electron microscopes with a magnification of several hundred thousand times, which makes us have a further understanding of the law of life activities of organisms. Most of the experiments stipulated in the biology syllabus of ordinary middle schools are done with microscopes, so the performance of microscopes is the key to do observation experiments well.

I. Optical System of Microscope

The optical system of microscope mainly includes objective lens, eyepiece, reflector and condenser. Broadly speaking, it also includes light source, filter, cover glass and glass slide.

(1) objective lens

The objective lens is the most important component that determines the performance of a microscope. It is installed on the objective lens converter, close to the observed object, so it is called objective lens or objective lens.

1, classification of objective lens

The objective lens can be divided into dry objective lens and immersion objective lens according to different use conditions; Among them, immersion objective lens can be divided into immersion objective lens and oil-immersed objective lens (the common magnification is 90- 100 times).

According to the different magnification, it can be divided into low-power objective (10 times or less), medium-power objective (about 20 times) and high-power objective (40-65 times).

According to aberration correction, it can be divided into achromatic objective lens (commonly used, which can correct the chromatic aberration of two colors in the spectrum) and apochromatic objective lens (which can correct the chromatic aberration of three colors in the spectrum, which is expensive and rarely used).

2. The main parameters of the objective lens:

The main parameters of the objective lens include magnification, numerical aperture and working distance.

① Magnification refers to the ratio of the size of the image seen by the eyes to the size of the corresponding specimen. It refers to the ratio of length, not the ratio of area. Example: The magnification is 100×, which refers to the specimen with the length of 1 μ m, and the length of the magnified image is100 μ m. If calculated by area, the magnification is 10000 times.

The total magnification of the microscope is equal to the product of the magnification of the objective lens and the eyepiece.

② Numerical aperture, also called aperture ratio, abbreviated as NA or A, is the main parameter of objective lens and condenser, which is directly proportional to the resolution of microscope. The numerical aperture of dry objective lens is 0.05-0.95, and the numerical aperture of oil-immersed objective lens (fragrant asphalt) is 1.25.

③ Working distance refers to the distance from the lower part of the front lens of the objective lens to the upper part of the specimen cover glass when the specimen is most clearly observed. The working distance of the objective lens is related to its focal length. The longer the focal length of the objective lens, the lower the magnification and the longer the working distance. Example: 10/0.25 and 160/0. 17 are marked on the objective lens, where 10 is the magnification of the objective lens; 0.25 is the numerical aperture; 160 is the lens barrel length (mm); 0. 17 is the standard thickness of cover glass (unit: mm). The effective working distance of 10x objective lens is 6.5mm, and that of 40x objective lens is 0.48mm.

3. The objective lens is first used to enlarge the specimen. The resolution is the most important factor that determines the performance of a microscope.

Resolution is also called resolution or resolution. Resolution is expressed by the value of resolution distance (the minimum distance that can be resolved between two objects). At the apparent distance (25cm), normal human eyes can see two objects with a distance of 0.073mm, and this value of 0.073mm is the resolution distance of normal human eyes. The smaller the resolution distance of the microscope, the higher the resolution, which means the better the performance.

The resolution of microscope is determined by the resolution of objective lens, which is determined by its numerical aperture and the wavelength of illumination light.

The resolution distance of the microscope is d=0.6 1λ/NA when using the ordinary central illumination method (bright illumination method that makes the light pass through the specimen evenly).

Where d is the resolution distance of the objective lens in nm.

λ —— wavelength of illumination light, in nanometer (nm).

Numerical aperture of objective lens

For example, the numerical aperture of oil-immersed objective lens is 1.25, and the wavelength range of visible light is 400-700 nm. If the average wavelength is 550 nm, d=270 nm, which is about half the wavelength of illumination light. Generally, the resolution limit of a microscope illuminated by visible light is 0.2 micron. ..

(2), eyepiece

Because it is close to the observer's eyes, it is also called eyepiece. Mounted on the upper end of the lens barrel.

1, structure of eyepiece

Usually, the eyepiece consists of two groups of lenses, the upper lens is called objective lens, and the lower lens is called convergent lens or field lens. There is an aperture between the upper and lower lenses or under the field lens (its size determines the field of view). Because the specimen is only imaged on the surface of the diaphragm, a short piece of hair can be stuck on the diaphragm as a pointer to indicate the target of a certain feature. Eyepiece micrometer can also be placed on it to measure the size of the observed specimen.

The shorter the length of the eyepiece, the greater the magnification (because the magnification of the eyepiece is inversely proportional to the focal length of the eyepiece).

2, the role of the eyepiece

It is to further enlarge the clear real image that has been enlarged by the objective lens, so that the human eye can easily distinguish it clearly.

The magnification of ordinary eyepiece is 5- 16 times.

3. The relationship between eyepiece and objective lens

The fine structure that has been clearly distinguished by the objective lens can't be seen clearly if it is not magnified by the eyepiece, and it can't reach the size that the human eye can distinguish; However, the fine structure that can't be resolved by the objective lens can't be seen clearly even if it is magnified by the high-power eyepiece, so the eyepiece can only play the role of magnification and will not improve the resolution of the microscope. Sometimes, although the objective lens can distinguish two close objects, it is still unclear because the image distance between the two objects is smaller than the resolution distance of the eyes. Therefore, the eyepiece and the objective lens are interrelated and mutually restricted.

(3), condenser

A condenser is also called a condenser. On the condenser bracket under the sample. It mainly consists of a condenser and an aperture. Among them, the condenser can be divided into bright field condenser (common microscope configuration) and dark field condenser.

1, main parameters of optical mirror

The numerical aperture (NA) is the main parameter of the condenser. The maximum numerical aperture is generally 1.2- 1.4, and the numerical aperture has a certain variable range. Usually, the number engraved on the upper lens holder represents the maximum numerical aperture. By adjusting the opening of the lower iris, various numerical apertures below this number can be obtained to meet the needs of different objective lenses. Some condenser lenses are composed of several groups of lenses, and the top group of lenses can be removed or moved out of the optical path, so that the numerical aperture of the condenser lens becomes smaller to adapt to the illumination when observed by a low-power objective lens.

2, the role of the condenser

The condenser is equivalent to a convex lens, which condenses light to enhance the illumination of the specimen. Generally speaking, the focal point of the condenser is designed to be about 65438 0.25mm above the lens plane at its upper end. (The focus is on the specimen to be observed, and the thickness of the slide is about 1. 1mm).

3. Variable aperture

Aperture, also called aperture, is located under the condenser and consists of more than a dozen pieces of metal with a round hole in the middle. Its function is to adjust the light intensity, so that the numerical aperture of the condenser is adapted to the numerical aperture of the objective lens. The larger the aperture, the larger the numerical aperture (after the observation, the aperture should be adjusted to the maximum).

Under the iris, there is also a circular filter bracket.

Note: Only the teachers' microscope (1600× or 1500×) in the middle school laboratory is equipped with a condenser, and the students' microscope (640× or 500×) is equipped with a rotating diaphragm. Close to the stage, a circular disk that can rotate in a circle, a rotating diaphragm (also known as a shutter), and circular holes with different sizes on the diaphragm are called apertures. The diameters are 2, 3, 6, 12 and 16 mm respectively. Rotate the rotating diaphragm, and each aperture on the diaphragm can be aligned with the aperture, and the intensity of light can be adjusted through different apertures.

(4) reflector

The reflector is a two-sided mirror that can rotate at will, with a diameter of 50mm, one side is flat and the other side is concave, and its function is to reflect the light coming from any direction through the light hole. The flat mirror has weak reflective ability, so it is used when the light is strong, and concave mirror has strong reflective ability, so it is used when the light is weak.

The reflector is usually a flat mirror on one side and a concave mirror on the other side, which is installed under the condenser and can rotate freely in both horizontal and vertical directions.

The function of the reflector is to make the light or natural light emitted by the light source shine on the condenser. When using a condenser, generally use a flat mirror, and use a concave mirror when not in use; Use a flat mirror when the light is strong, and use a concave mirror when it is weak.

After the observation, the reflector should be placed vertically.

(5) Lighting source

Microscopes can be illuminated by natural or artificial light sources.

1, natural light source

Light comes from the sky, preferably reflected by white clouds. Do not use direct sunlight.

2. Artificial light source

① Basic requirements for artificial light source: sufficient luminous intensity; The light source cannot generate too much heat.

② Common artificial light source: microscope lamp; fluorescence

(6) optical filter

Installed between light source and condenser. The function is to let the light in the selected band pass and absorb other light, that is, to change the spectral composition of light or weaken the intensity of light. Divided into two categories: optical filters and liquid filters.

(7) Cover glass and glass slide

The surface of cover glass and glass slide should be very flat, without bubbles and scratches. It is best to choose colorless and transparent ones, and wash them before use.

The standard thickness of cover glass is 0.17 0.02 mm. If cover glass is not used or the thickness of cover glass is not appropriate, the imaging quality will be affected.

The standard thickness of the slideway is 1. 1.04 mm, and the general usable range is1-1.2 mm. Too thick will affect the efficiency of the concentrator, and too thin will easily break down.

Second, the mechanical device of the microscope

The mechanical device of microscope is an important part of microscope. Its function is to fix and adjust the optical lens, fix and move the specimen, etc. It mainly consists of a lens frame, a lens arm, an object stage, a lens barrel, an objective lens converter and a focusing device.

(1), mirror base and mirror arm

1, the role of the mirror base is to support the whole microscope, equipped with mirrors, and some are also equipped with lighting sources.

2. The function of the mirror arm is to support the lens barrel and the stage. There are two types: fixed and tiltable.

② Stage (also known as workbench and mirror stage)

The stage is used for placing slides, and it has two shapes: round and square, in which the square area is120mm×110mm. There is a lamp hole in the center, and there are two small holes for installing flat clips on the left and right sides behind the lamp hole. There are two kinds: fixed and mobile. Some stages are equipped with cursors in vertical and horizontal coordinates, with a general reading of 0.1mm. The cursors can be used to measure the size of the specimen and mark the inspected parts.

(3), lens barrel

An eyepiece is placed at the upper end of the lens barrel, and the lower end is connected with an objective lens converter. There are two types: fixed and adjustable. The length of the mechanical lens barrel (the distance from the upper edge of the eyepiece barrel to the lower end of the screw hole of the objective lens converter is called the lens barrel length or the mechanical lens barrel length) is called the fixed lens barrel, and the adjustable lens barrel can be replaced. Most new microscopes use fixed lens barrel, and most domestic microscopes also use fixed lens barrel. The mechanical lens barrel length of domestic microscopes is usually 160 mm.

There are two kinds of lens barrels for installing eyepieces: single barrel and double barrel. Single cylinder can be divided into vertical and inclined type, and double cylinder is inclined type. Among them, binocular microscope can observe both eyes at the same time, reducing eye fatigue. The distance between the two lens barrels can be adjusted, and one of the eyepieces has a diopter adjustment device, which is convenient for observers with different eyesight.

(4), objective lens converter

The objective lens converter is fixed at the lower end of the lens barrel and has 3-4 objective lens screw ports. The objective lenses should be arranged in the order of magnification. When rotating the objective lens converter, you should hold the rotating disk with your fingers and rotate it. Don't push the objective lens with your fingers, because pushing it for a long time will easily skew the optical axis and deteriorate the imaging quality.

(5), focusing device

The microscope is equipped with a coarse focusing screw and a fine focusing screw. Some microscopes have coarse quasi-focus spirals mounted on the same axis, with the large spiral being the coarse quasi-focus spiral and the small spiral being the fine quasi-focus spiral. Other separate arrangements. A pair of larger screws located at the upper end of the lens arm are coarse focus screws, and the lens barrel rises or falls 10mm every rotation. A pair of smaller screws located below the coarse focus screw are fine focus screws. When it rotates once, the lifting value of the lens barrel is 0.65438 0.0 mm, and the focusing range of the precision focus screw is not less than 65438 0.8 mm. ..

Third, the use of microscope and its components

1. When using a monocular microscope, you should get into the habit of observing with your left eye (because you usually draw with your right hand). When observing, open your eyes at the same time, don't close one, because it is easy to get tired. In order to train students to open their eyes at the same time, you can cut a rectangular piece of hard paper about 14cm long and 6cm wide, dig a round hole with a diameter slightly smaller than its outer diameter at the upper end of the lens barrel near the left end, put the round hole on the upper section of the lens barrel, open your eyes at the same time during observation, and block the line of sight of the right eye with the right end of the paper. After a period of training, we can get used to opening our eyes at the same time and then taking the paper away.

2. The joint between the mirror arm and the mirror base of the straight tube microscope is a mechanical joint, which can be used to adjust the inclination of the lens barrel for easy observation. The lens arm should not be tilted back too much, generally not more than 40. However, when using temporary loading observation (when the lens barrel tilts, the stage tilts, and the liquid on the slide is easy to flow out), it is forbidden to use tilting joints, especially when loading acidic reagents, so as not to pollute the lens body.

3, the use of eyepiece and objective lens

Generally, the eyepiece with medium magnification (10×) and the objective with the lowest magnification are used to start observation, and the objective with higher magnification is gradually used to find the magnification that meets the experimental requirements.

When replacing the objective lens, first observe it with a low-power lens and adjust it to the correct working distance (the imaging is clearest). If the high-power objective lens is further used for observation, before replacing the high-power objective lens, the part of the object image that needs to be enlarged for observation should be moved to the center of the field of view (when replacing the low-power objective lens with a high-power objective lens for observation, the object image range in the field of view is much reduced). The low power objective and the high power objective are basically focused (focused at the same height). When the low-power objective lens is used to observe clearly, you can see the object image by replacing the high-power objective lens, but the object image is not necessarily clear, so you can adjust it by turning the fine-tuning focus screw.

Generally speaking, when using any objective lens, the upper limit of effective magnification is 1 1,000 times its numerical aperture, and the lower limit is 250 times its numerical aperture. If the numerical aperture of the 40× objective lens is 0.65, the upper and lower limits are 1000×0.65=650 times and 250×0.65≈ 163 times respectively, and those exceeding the upper limit of the effective magnification are called invalid magnification, which cannot improve the observation effect. The magnification below the lower limit cannot be distinguished by human eyes, which is not conducive to observation. Generally, the most practical amplification range is 500-700 times the number between numerical apertures.

4, the use of oil-immersed objective lens

When using oil-immersed objective lens, the same height focusing is generally not used. Focusing at the same height is only applicable to the original objective lens of each microscope, which is a very favorable and convenient condition when using low-power and high-power objective lenses, but it is limited when using oil-immersed objective lenses. Generally speaking, it is safe to focus on the same height when observing the specimen (slide) without cover glass with oil mirror, but it should be used with caution for the specimen with cover glass, because the working distance of the oil-immersed objective lens is very short.

When the objective lens is immersed in oil, only aromatic tar drops on the specimen. After the observation, the cleaning work should be done in time. If it is not done in time, the fragrant tar will stick to dust, and dust particles may wear the lens when wiping. When the fragrant tar is exposed to the air for a long time, it will become thick and dry, which is very difficult to wipe, which is very unfavorable to the instrument. Wipe carefully. Wipe the front end of the oil-immersed objective lens once or twice with dry mirror paper to remove most of the oil stains, then wipe it twice with xylene dripping wet mirror paper, and finally wipe it again with dry mirror paper. The fragrant asphalt on the specimen can be wiped off by "paper-pulling method" (that is, a small piece of mirror-wiping paper is covered on the fragrant asphalt, then some xylene is dropped on the paper, and the paper is pulled out while being wet, so that it can be wiped continuously for three or four times, and generally the smear specimen without cover glass will not be damaged). Mirror cleaning paper should also be dustproof. Generally, before use, each page is cut into 8 small pieces and stored in a clean Petri dish, which is economical and convenient to use.

5. How to use the condenser

(1), the reason for using the condenser

When the magnification increases, on the one hand, the higher the magnification, the more lenses and the more light absorbed by the lenses; On the other hand, the brightness of the field of view (referring to the range where the specimen can be seen) is inversely proportional to the square of the magnification, that is, the higher the magnification, the darker the field of view. In order to get enough brightness, a condenser must be installed to focus the light on the specimen to be observed.

(2) Observe the height of the condenser.

When observing, in order to ensure the best observation effect, the focusing focus of the condenser should just fall on the specimen. In order to achieve this condition, it is necessary to adjust the height of the condenser. When irradiated with parallel light, the focus of the condenser is about 65438 0.25mm above the center of the upper lens plane. Therefore, when observing, it is often necessary to raise the condenser to a height where the upper plane of the lens is only slightly lower than the plane of the stage, so that the focus may fall on the specimen on the standard thickness slide. When using a glass slide thinner than the standard thickness to hold the specimen, the position of the condenser should be lowered accordingly, while when using a glass slide with too thick thickness, the focusing point can only fall below the specimen, which is not conducive to careful observation.

③ Coordination of condenser and objective lens

The so-called cooperation here is to make the numerical aperture of condenser and objective lens consistent, so as to make better and more detailed observation. If the numerical aperture of the condenser is lower than that of the objective lens, part of the numerical aperture of the objective lens is wasted, so that the highest resolution cannot be achieved. If the numerical aperture of the condenser lens is larger than that of the objective lens, on the one hand, the specified resolution of the objective lens cannot be improved, on the other hand, the clarity of the object image will be reduced due to the wide illumination beam. The operation method of matching the condenser with the objective lens is: after focusing, remove the eyepiece lens barrel, close the aperture under the condenser to the minimum, and then slowly open it. Open it to the same caliber as the diameter of the field of view, and then press the eyepiece to observe. Every time the objective lens is replaced, the matching operation should be carried out in turn. Some condenser aperture borders are engraved with scales indicating the aperture, which can be matched according to the scales.

The invention of microscope and every innovation of microscope in history have brought great progress to human cognition. It has brought unprecedented expansion to human life. Today, driven by scientific and technological innovation, the use of microscope has become a basic skill for middle school students. Mastering the structure, using it scientifically and maintaining it well will make it a window for teenagers to explore the future world.