Device of a microscope

Measurement of value of microscoping object.

Microscope - optical instrument intended for obtaining of the increased maps of small objects, invisible by a unaided eye. A normal eye of the person apart of best vision (25 cm) can distinguish a minute structure consisting of lines or points provided that they are from each other apart not less than 0,07 mm.

The sizes of bacterias, organic cages, small-sized chips etc. are much less than this value. For detection and the analyses of such objects will be used different kinds of microscopes. The optical scheme of a microscope consists of two parts: objective and eyepiece (fig. 1). The objective introduces a system of short-focus lenses. The eyepiece of a microscope consists of several lenses. The path of rays in a microscope is showed on fig. 2.

The considered object AB placed near to the main focus of an objective, forms the valid, return, increased image A₁B₁ behind an objective. This image is located between focus and the optical center O₂ of an eyepiece. By consideration of this image in the eyepiece, as in a magnifying glass, it will be even more increased, imaginary and direct. In the final accounting microscope gives the image A₂B₂, which one is a converse in relation to a subject an AB. The linear increase of a microscope N_M is equal to product of increases produced by a objective and the eyepiece:

(1)

The increase of a microscope can not be as much as large, and its value does not exceed 3000. It is connected to restricted permitting ability of a microscope, conditioned by diffraction phenomena, as the image of any object is outcome of diffraction and interference by dispersed object of light. Properties of an optical system to give the separate images of two close arranged luminous or lighted points of object (member of frame) is called permitting ability. Permitting ability is accepted for measuring by value:

R = 1/ℓ (2)

where ℓ- permitting limit, i.e. least possible spacing interval between two points, at which one they are visible separately.

Applying this concept to conditions of microscoping of biological objects, it is possible to consider, that the permitting limit causes the least value of those structural parts, which one can differ in a drug. As it is visible from the formula (2) restricted permitting ability of a microscope the is more, than its permitting limit less. Is theoretically demonstrated, that the permitting limit can be defined under the formula (3).

ℓ=0,5 λ/nsin(α/2) (3)

Where λ - wavelength of light, n -refractive index of medium, between a subject and objective, sin(α/2) - aperture angle, i.e. angle derivated by extreme rays, falling in a lens.

Product nsin(α/2) – is called an angular aperture A.

Apparently, than above permitting ability, especially small-sized parts is possible to consider. The increase of permitting ability of the optical microscope is reached by reduction of a wavelength of light, with the help which the research is made, for example, by application of a ultraviolet radiation (for this purpose the special microscope) and increase of a numbered aperture of a microscope is applied. For what it is necessary:

1. To use an immersion objective, for which the space between an observable subject and an entrance lens is filled by a liquid with a parameter of refraction equal to a parameter of refraction of glass (air n = 1, glycerinum n = 1.45)

2. To use an objective with a large aperture angle. The value of an aperture angle is indicated on an objective. For example, increase is equall 8, then the aperture angle is equall 0,2.

3. In figure l are figured a) a general view and b) an optical system of a biological microscope. Its main parts: the basis, box with the micrometric gear 9, subject stage 10, revolver 11 with lenses 5, condenser 2 and eyepiece 7.

The optical scheme consists of 2 parts: lighting and observation. In a lighting part enters: a mirror 1, condenser with an iris diaphragm 3 and volumetric light filter 4. In observation -objective, prism 6, eyepiece, combined in a radiographic cone of a microscope.

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