The type of elect on microscope developed by Ruska is called the transmission microscope. The object to be examined is in the form of a thin section. The electron beam goes right through this in the same way that light pierces the object in a light microscope. There are, however, several other types of electron microscope, the most important apart from the transmission microscope being perhaps the scanning electron microscope. The secondary electrons emitted are collected by a detector and the current is recorded.
Magnetic coils cause the electron beam to scan the object in the same way as the beam of a TV tube.
The variations in the emission of secendary electrons carn be used to build up an image. The advantage is the large depth of focus which gives a three-dimensional image as opposed to the sectional image obtained with a transmission microscope. However, the resolution is poorer. These two types of microscope thus complement each other. Electron microscopy has developed extremely over the last few decades, with technical improvements and entirely new designs.