Here is an article about the basic concepts of light microscopy at University College London, Department of Geography. It discusses the different kinds of light microscopy, taking good care of your microscopes, and some procedures on how to use microscopes and the use of oil immersions. Light microscopy includes dark field, phase contrast and bright field microscopes.

The simplest of all the optical microscopy illumination techniques is the bright field microscopy. It uses light from an incandescent source which is aimed toward a lens beneath the stage called the condenser, through the specimen, through an objective lens, and to the eye through a second magnifying lens, the ocular or eyepiece. Most microscopes have a built-in illuminator. The condenser is used to focus light on the specimen through an opening on the stage. After passing through the specimen, the light is displayed to the eye with an apparent field that is much larger than the area illuminated. The magnification of the image is simply the objective lens magnification typically stamped on the lens body times the ocular magnification. The magnification is only limited by the magnifying power of the lens system. The limit of magnification for most light microscopes is 1000 times which is set by an intrinsic property of lenses called resolving power.

The most common use of the microscope involves the use of a specimen mounted to a microscope glass slide. This works well when the specimens are stained with bright colors. You can use an oil when you place a glass cover over the specimen, sample staining methods such as Methylene blue, Safranin, Crystal violet and differential stains Negative stains, flagellar stains, endospore stains, they are colored usually blue or you can use a polarizing filter on the light source to highlight features not visible under white light, and use of filters especially for mineral samples are the different techniques on viewing and enhancing specimen under a bright field microscope.

General components of a bright field microscope are the: base – a supporting structure that usually contains an electrical light source or illuminator; objective lens- to magnify the image; oculars – to magnify the image from the objective lens, arm - to support the structure that connects the lens systems to the base; body tube – it sends light to the ocular lens; condenser lens – it directs light to pass through the specimen; stage – a platform that allows mechanical movement of a microscope slide; adjustment knobs – to course and fine focus adjustment.
A microscope with one ocular lens is often called a monocular; a microscope with two oculars is called a binocular.

Steps when using the bright field microscope:

1. Mount the specimen on the stage
2. Optimize the lighting
3. Adjust the condenser
4. Focus, locate, and center the specimen
5. Adjust eyepiece separation and focus
6. Select an objective lens for viewing
7. Adjust illumination for the selected objective lens

The advantage of a bright field microscope is the simplicity of setup with only basic equipment required, no sample preparation required, bright field microscope allows viewing of live cells. And it gives incredible views of algae, desmids and some prepared slides.
But it has also limitations that it is a very low contrast of most biological samples, and low apparent optical resolution due the blur of out of focus material, and the lamp. Original article