Three basic concepts form the foundation for the present-day scientific description of a vast field of optical phenomena, among them the occurrence of the spectral colours as a result of light passing through a transparent medium of prismatic shape. They are: ‘optical refraction’, ‘light-ray’, and ‘light-velocity’ – the latter two serving to explain the first. In a science of optics which seeks its foundation in the intercourse between man’s own visual activity and the doings and sufferings of light, these three concepts must needs undergo a decisive change, both in their meaning and in their value for the description of the relevant optical phenomena. For they are all purely kinematic concepts typical of the onlooker-way of conceiving things – concepts, that is, to which nothing corresponds in the realm of the actual phenomena.