Special types of microscopes
Dark ground microscope:
It is also called dark field illumination microscope. There are certain microorganisms, which are very difficult to stain example spirochetes. To visualize them under microscope dark field illumination is used. The microorganism appears bright against a dark background. It is similar to dust particle seen in a beam of light from a ventilator in a dark room. In this microscope a special condenser with a central black area is placed just behind the objective. A dark ground, phase contrast microscope can be made from an ordinary microscope. For this, cut out a thick talc sheet of the size of a filter. Colour the central two third with black ink. Place it along the filter in the holder below the condenser
Certain dyes have the characteristic of glowing when exposed to ultraviolet light. In fluorescent microscope the object is stained with these (fluorochrome) dyes. The light source of microscope is replaced with the source that provides only ultraviolet light. The object appears as a glowing particle against a dark background. Rhodamine and Auramine are commonly used fluorochrome dyes. If an antibody is attached to these fluorochrome dyes, the presence of specific antigen can be detected. This is called Immunofluorescent microscopy
Phase contrast microscope:
This microscope is used for observing unstained living organism with good contrast and high resolution. It is useful for the study of structure of large microorganisms, tissues and cells. Unstained bacteria and cell consist of alternate strips of material of different refractive indices that cause the light to acquire small phase differences. These differences are exaggerated by causing the direct and diffracted rays to pass through different thickness of glass in phase plate. Direct and diffracted light beams are then recombined to produce an image
This microscope is used to see viruses or parts of cells smaller than the limits of resolution of the light microscope. It utilises a beam of electrons instead of visible light and electromagnetic fields in place of optical lenses. An object forms an image in electron microscope because its solid content scatters the electron beam and so casts a shadow in the electron beam. The image cannot be seen with eye. Instead, it is focused on a screen and/or is photographed. Further magnification and resolution can be obtained by enlarging the photographs
Originally published at https://dooogly4u.blogspot.com.