The existence of light pressure and its magnitude are of great importance for the various 18th century theories about the physical nature of light. This paper examines the observations and experiments conducted in the 18th century to detect and measure this pressure. The reception of these results reveals another scientific strategy besides confirmation and refutation, namely, estimation of parameters. The author proposes the notion of primary evidence, for phenomena that are taken to confirm or refute theories, and problematic phenomena, for phenomena to which accepted theories are applied.