By the end of the 19th century, optical microscopy had hit the wall. Objectives had been made sufficiently good to achieve the theoretical maximum resolution defined by the wavelength of light. There seemed to be no obvious way forward. One approach was to reduce the wavelength, and ultraviolet microscopes had a certain vogue in the first half of the 20th century. The problems with these were many; recording the image on film is easy, but focusing on a fluorescent screen is not so easy. Lenses, and coverslips, had to be made of quartz, because most glass does not transmit far enough into the UV to be useful. Without the range of special glasses available for the visible range, lens correction was not so good. And cells were always going to be dead — at least pretty soon! So UV microscopy was essentially a technique for fixed material.