Microscopes Anatomy 101
You must understand the basic anatomy of the various kinds of microscopes in order to evaluate them and what you need.
Coarse Focus Knob
This knob provides for quick focus on both high-powered and low-powered microscopes. It operates on a rack and pinion system that can potentially lower the objective to touch the specimen and damage both. A safety rack stop can be set to prevent this.
This is the lens closest to the object or specimen. Usually a microscope has a variety of objective lenses to allow for multiple magnifications. These should always be made of glass - plastic is quite inferior, even for beginner microscopes. The total magnification is the eyepiece magnification multiplied by that of the objective lens.
Simply enough, the specimen stage is where the slide containing your specimen will be positioned. Below it is a light source; directly above the slide is the objective lens. Care should be taken to not let the objective lens go low enough to touch the specimen stage.
This is a concave or adjustable flat mirror that reflects an external light source into the microscope. The flat side provides a better image, but you should use the concave side for higher magnifications or brighter light.
The base is the heavy, bottom part of a microscope, typically shaped to make room for the light source.
Stage Holder Clamp
Because magnification greatly increases even the slightest movement, microscopes require a clamp to hold down the slide housing the specimen. Trying to view an object without one would likely prove difficult.
Safety Rack Stop
As mentioned in the "coarse focus knob", this feature is a mechanism that stops the pinion from rolling along the rack, lowering the objective too close to the specimen. It is imperative to set this properly to prevent damage to your equipment.
Like the base, the arm is a functional, heavy component that provides stability to the microscope and houses the stage, body, eyepieces, etc.
Fine Focus Knob
High-powered microscopes need a fine focus knob to take over where the coarse focus knob left off. This moves your objective lens (or specimen stage) along a rack and pinion, just like the coarse knob. However, because the movement is slight, there is less risk of crushing your objective against the specimen.
This is the housing of the various objective lenses that a microscope has. It is a rotating turret (for microscopes with more than one objective lens) that allows for quick changes in magnification.