Microscopists always want to look more closely, but up until quite recently biologists have been drastically restricted in what they can study with the light microscope. For a microscope used in traditional fashion, the laws of physics dictate that two objects need to be more than 200nm apart (i.e. about half the wavength of the illuminating light) or their images will blur together. This is fine for viewing cells – for example, a red blood cell is about 7000nm in diameter - but for many of a cell's most interesting features the so-called “diffraction barrier” does pose problems. The plasma membrane for example is about 7 to 10 nm thick, and cannot be resolved with light microscopy. These finer structures can be examined with electron microscopy, where the resolving power increases to less than a nanometre, but with this approach biologists cannot take advantage of a wide range of powerful specimen preparation techniques, most particularly the targeting of structures with fluorescent probes.