The light microscope and the electron microscope refers to the type of microscope that is used to view the non- living specimen. The non-living specimen is usually placed in a slide.
They are so very small, that they can not be seen even with the strongest microscope. And long ago, microscopes were very simple compared to days'. Only when things didn't add up, did they begin to think that there must be other things out there. The word means that these are somethings that can not be cut. But even now we know that has changed. (atomic weapons are based on 'cutting' or splitting atoms).Here is a link that may help: http://atomictimeline.net/index.php