Bright Field Microscope
The most commonly used microscope for general laboratory observations is the standard bright field microscope (Figure 2.6). It contains the following components
· A mirror or an electric illuminator is the light source which is located at the base of the microscope.
· There are two focusing knobs, the fine and the coarse adjustment knobs which are located on the arm. These are used to move either the stage or the nosepiece to focus the image.
· Mechanical stage is positioned about half way up the arm, which allows precise contact on moving the slide.
· The substage condenser is mounted within or beneath the stage and focuses a cone of light on the slide. In the simpler microscope, its position is fixed where as in advanced microscope it can be adjusted vertically.
The upper part of microscope arm holds the body assembly. The nose piece and one or more eyepieces or oculars are attached to it. The body assembly contains series of mirrors and prisms so that the barrel holding the eyepiece may be tilted for viewing. Three or five objectives with different magnification power are fixed to the nosepiece and can be rotated to the position beneath the body assembly. In bright field microscopy; the specimen is viewed against a bright background. The details of the image are defined by the surrounding light. A series of finely ground lenses forms an image which is many times larger than the real image. This magnification occurs when light rays from an illuminator (light source), pass through a condenser which has lenses that direct the light rays through the specimen. The light rays then pass into objective lens (the lens closest to the specimen). The image is again magnified by the ocular lens or the eyepiece. (Figure 2.7).
· Magnification is the process of enlarging the image of the specimen and can be calculated by multiplying the objective lens magnification power by ocular lens magnification power.
Representative magnification values for a 10X ocular are:
Scanning objective (4X) × (10X) = 40X magnification
Low power objective (10X) × (10X) = 100X magnification
High dry objective (40X) × (10X) = 400X magnification
Oil immersion objective (100X) × (10X) = 1000X magnification
Oil immersion lens is designed to be in direct contact with oil placed on the cover slip. An oil immersion lens has a short focal length and hence there is a short working distance between the objective lens and the specimen. Immersion oil has a refractive index closer to that of glass than the refractive index of air, so the use of oil increases the cone of light that enters the objective lens. Figure 2.8 explains the working principle of oil immersion objective lens.