A special accessory required for dark-field observation is a dark-field spotting scope. It is characterised by not allowing the light beam to pass through the examined object from bottom to top, but by changing the path of the light so that it is directed obliquely towards the examined object, so that the illuminating light does not enter directly into the objective lens, and a bright image is formed by using the reflected or diffracted light from the surface of the examined object. The resolution of dark-field observation is much higher than bright-field observation, * up to 0.02-0.004

The basic principle of phase contrast microscopy is that the difference in optical range of visible light transmitted through a specimen is turned into a difference in amplitude, thus increasing the contrast between various structures and making them visible. The light is refracted through the specimen and deviates from the original light path, while being delayed by 1/4λ (wavelength). If the 1/4λ is increased or decreased again, the optical range difference becomes 1/2λ, and the interference between the two beams of photosynthesis is strengthened after the axes of the two beams are interfered with, and the amplitude increases or decreases, thus improving the contrast. In the structure, phase contrast microscope has two special features which are different from ordinary optical microscope:

1. annular diaphragm (annulardiaphragm) is located between the light source and the condenser, the role is to make the light through the condenser to form a hollow cone of light, focusing on the specimen.

2. phase plate (annularphaseplate) in the objective lens coated with magnesium fluoride phase plate, direct or diffracted light can be delayed phase 1/4λ. There are two kinds:

1.A phase plate: the direct light delayed 1/4 λ, two groups of light waves co-axial light wave addition, amplitude increase, the specimen structure than the surrounding medium is brighter, the formation of bright contrast (or negative contrast).

2.B phase plate: the diffracted light is delayed by 1/4 λ, the two groups of light waves after the merging of the axis of the light wave is reduced, the amplitude becomes smaller, the formation of dark contrast (or positive contrast), the structure is darker than the surrounding medium.