Complex sentence: It remains the key to measurement for much of modern science and technology but at the end of the 19th Century scientists discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain.
Split into simpler sentences: It remains the key to measurement for much of modern science and technology. But at the end of the 19th century scientists discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain.