Virtual microscopes have only been possible since the advent of fast internet connections allowing rapid transfer of images to a browser window. The virtual microscope system works by presenting small parts of a large image that has been diced electronically into many pieces, an approach used to facilitate zooming and panning in many web and phone-based map applications. The diced images are not simply equal mosaic pieces of the whole image; the dicing software creates pieces at several resolutions, so the viewer can both pan around the image and zoom in and out. Dicing the large images of microscope slides is undertaken using a drawing package – but it's automated so they are not cut by hand! The resulting file structure is a pyramid, rising from a single low resolution image to many hundreds of images at the highest resolution. The browser viewer only loads the image of the area required at the zoom level specified, making for rapid loading, viewing or moving and giving the impression of zooming and panning.
If you would like to know more about the features you see in the virtual microscope, or how geologists identify and classify rocks and minerals, there are lots of resources available.