These lectures present a physiological account of human colour vision and its disorders, beginning with retinal anatomy and the role of visual purple in rods and cones to convert light into neural impulses. The author outlines a retino-cerebral framework in which photochemical changes produce impulses whose character varies with wavelength, and argues that deficiencies in the colour-perceiving apparatus produce forms of colour-blindness ranging from dichromacy to reduced trichromacy. The text surveys entoptic phenomena, visual acuity and the evolution of the colour sense, then offers practical guidance for detecting colour-vision defects, describing test principles and instruments such as lantern and plate tests.