This final volume of papers by Lord Rayleigh covers the period from 1911 to his death in 1919. The first of the Solvay Conferences in 1911 played a key role in the foundation of quantum theory. Although invited, Rayleigh did not attend. His principal achievements lay in development and consolidation across classical physics, in which he continued to work. In a 1917 paper, he used electromagnetic theory to derive a formula for expressing the reflection properties from a regularly stratified medium. In 1919, he investigated the iridescent colours of birds and insects. Rayleigh continued his long-standing participation in the Society for Psychical Research, founded in 1882 for the study of 'debatable phenomena'. One of his last publications was his presidential address to that society, which considers several highly unorthodox views and practices. He concludes by asserting the importance to scientists of maintaining open minds in the pursuit of truth.