与可见光不同，电子是带电粒子，不能用光学透镜汇聚成像，但电子可以凭借轴对称的非均匀电场、磁场的力使其汇聚，从而达到成像的目的。人们把用静电场做成的透镜称为静电透镜，把用非均匀轴对称磁场做成的透镜称为短磁透镜。
短磁透镜与静电透镜相比，具有以下优点。
（1）改变线圈中的电流强度，可方便地控制透镜焦距和放大倍数；而静电透镜则必须花费很高的加速电压才能达到此目的。
（2）短磁透镜中线圈电流的电源电压通常为60～100V，不用担心击穿；而静电透镜的电极则需要加上数万伏的电压，容易造成击穿。
（3）短磁透镜的像差较小。
目前，电子显微镜主要采用短磁透镜使电子成像，只在电子枪和分光镜中才能使用静电透镜。电子显微镜与光学显微镜的比较见表3-2。(www.xing528.com)
表3-2 电子显微镜与光学显微镜的比较
光学显微镜与电子显微镜的差异用英文描述如下。
Why use electrons instead of light?
A modern light microscope（often abbreviated to LM）has a magnification of about 1000x and enables the eye to resolve objects separated by 0.0002 mm.In the continuous struggle for better resolution，it was found that the resolving power of the microscope was not only limited by the number and quality of the lenses but also by the wavelength of the light used for illumination.It was impossible to resolve points in the object which were closer together few hundred nanometers.Using light with a short wavelength（blue or ultraviolet）gave a small improvement；immersing the specimen and the front of the objective lens in a medium with a high refractive index（oil）gave another small improvement but these measures together only brought the resolving power of the microscope to just under 100 nm.
In the 1920s it was discovered that accelerated electrons behave in vacuum just like light.They travel in straight lines and have a wavelength which is about 100，000 times smaller than that of light.Furthermore，it was found that electric and magnetic fields have the same effect on electrons as glass lenses and mirrors have on visible light.Dr.Ernst Ruska at the University of Berlin combined these characteristics and built the first transmission electron microscope（often abbreviated to TEM）in 1931.For this and subsequent work on the subject，he was awarded the Nobel Prize for Physics in 1986.The first electron microscope used two magnetic lenses and three years later he added a third lense and demonstrated a resolution of 100 nm，twice as good as that of the light microscope.Today，using five magnetic lenses in the imaging system，a resolving power of 0.1 nm at magnifications of over 1 million times can be achieved.
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