In Charlotte Brontë’s Shirley (1849), the narrator remarks of the eponymous heroine: ‘Her nature is in her eye …’ (Brontë [1849] 2008, 326). Specifically, throughout the text, Shirley’s eyes are described as flashing; Caroline’s, meanwhile, are described as glowing. While Brontë’s interest in and exploration of phrenology is well-documented, Shirley’s focus on the eyes is quintessentially physiognomic. This article explores the two major sequences that describe the heroines’ flashing and glowing eyes in relation to Brontë’s use of physiognomic principles and her ideals of gender. I argue that Brontë’s portrayal of Shirley and Caroline not only questions the allegedly unifying power of physiognomy but also gives it a phrenological treatment in which the external signifier does not always match the internal signified. Furthermore, the specific imagery of flashing and glowing is emblematic of Shirley’s refusal to adhere to binaries. While flashing ‘reads’ as masculine and glowing ‘reads’ as feminine, Brontë’s conception of Shirley and Caroline demonstrates that masculinity and femininity, far from being a binary, work on a scale.