（Flat window）The lens of microscope includes eyepiece and objective lens. The eyepiece is mounted on the upper end of the lens barrel, close to the human eye. The objective lens is mounted on a converter at the lower end of the lens barrel.
(1) （Flat window）Types of objective lens and identification of marks on the metal shell of objective lens
Objective lens is the most important optical component of microscope. Its function is to magnify the object for the first time and determine the image quality and resolution. The resolution mainly depends on the numerical aperture NA (aperture ratio).
The types of objective lens can be divided into the following categories according to the degree of aberration correction:
① （Flat window）Achromatic objective
A common objective lens, whose shell is not engraved with a symbol. This objective lens can focus red and blue light in the spectrum at one point, and yellow and green light on another point. The best resolution range is 510-630nm, and its performance is poor, so it is not suitable for photography.
② （Flat window）Apochromatic objective
Apo is engraved on the shell of the objective lens, which has high performance. It can focus the red, blue and yellow light in the visible spectrum, and correct the spherical aberration and other aberrations of the red and blue light. The best clear range is 400-720 nm, which can hold all visible spectrum. Suitable for observation and any color light.
③ （Flat window）Fluorite objective is also called half apochromatic objective
FI is engraved on the shell of the objective lens. The correction of chromatic aberration is between achromatic objective and apochromatic objective, and the best clear range is from 430 nm to 680nm. Including most of the visible spectrum, suitable for observation or photography with various filters.
The results show that the images of the center and the edge of the field of vision can not be focused at the same time. The center is clear, the edge is blurred or opposite, and increases with the increase of magnification.
④ （Flat window）Flat field objective
The shell of the objective lens is engraved with PI. The flat field objective lens not only has the advantages of common objective lens, but also corrects the bending of image field. There is no phenomenon that the center and edge of the field of view are not in focus at the same time. Because the image is flattened, the image is larger than that of ordinary objective lens under the same multiple, which is the ideal objective lens today. There are many kinds of flat field objective, among which "Pi apo" (flat field achromatic objective) is the best. The eyepiece should be used with the special flat field.
Understanding of a set of data representing performance on objective lens
Traditionally, the objective with magnification of 10 times or less is called low power objective, the objective with magnification of 40-45 times is called high power objective, and the oil immersed objective with magnification of 90-100 times is called oil lens. A group of numbers are often marked on the general objective lens to indicate the relevant performance indexes of the microscope, which are 10 / 0.25 and 160 / 0.17 on the low power objective, 40 / 0.65 and 160 / 0.17 on the high-power objective, and 100 / 1.25 and 160 / 0.17 on the oil lens. Where 10 (40, 100) is the magnification; 0.25 (0.65, 1.25) is the numerical aperture (aperture ratio); 160 or 170 (mm) is the length of the tube; 0.17 (mm) is the thickness of the required cover glass.
(2) Type of eyepiece
The function of eyepiece is to enlarge the image of objective lens for the second time, without increasing the resolution, but can correct the remaining aberration and chromatic aberration of objective lens. It is generally marked with 5x, 7x, 10x, 15x, 16x and other magnification, which can be selected according to needs.
There are two types of eyepiece diameter: standard type and wide field type
