This is an analysis of the optical revolution in the context of early 20th-century Britain. Far from merely involving the replacement of one optical theory by another, the revolution also involved substantial changes in instruments and the practices that surround them. People's judgements about classification, explanation and evaluation were affected by the way they used such optical instruments, such as spectroscopes, telescopes, polarizers, photometers, gratings, prisms and apertures. The two instrumental traditions in this historical period each nurtured a body of practice that exemplified how optical instruments should be operated, and especially how the eye should be used. These traditions function just like paradigms, shaping perspectives and even world views. This book should be of interest to scholars and graduate students in the history of science, history of instruments, philosophy of science and science studies. It can also be used as a textbook in graduate courses on 19th-century physics.