Google Books, in partnership with many of the great libraries in Europe, has just released full-color scans of important texts from the 16th and 17th centuries. They include Nostradamus’ prophecies, Kepler’s textbook on astronomy, and several works by Galileo including his Systema cosmicum, arguing that the Earth revolves around the Sun. The full-color scans are particularly important for illustrations, diagrams, maps, and distinguishing handwriting. Some of the books ages’ lead to bleedthrough (where the type on the other side of the page is visible), but even in those cases the full-color images give a sense of how the physical material has changed over time. There is not a separate interface through which you can access these books, but using the date limiters in the Google Books Advanced Search will help you find them. More detail is available at the Inside Google Books blog post.