A principal concern of the author in writing these articles has been to elucidate the conceptual structures that underlie the scientific thought of the Middle Ages - the philosophical and cultural assumptions, presuppositions and motivations that determine the way concepts are formed and questions are answered. In the first group of articles Professor Eastwood focuses on astronomy in Latin Europe in the 5th-11th centuries, looking especially at the use, development and interpretation of diagrams in works on planetary motion. The following studies turn to optics and visual theory. They examine Robert Grosseteste's views on the rainbow, refraction and empirical knowledge, and study specific instances of how medieval thinkers, both in the Latin and Islamic worlds, reinterpreted and reformulated the concepts they had inherited.