Riley’s paintings of the 1960s were the most famous works of what became called Op art. This label referred to the optical effects that dominate the viewer’s experience of the work. Sometimes such Op art works have an almost physical effect, destabilising the viewer. In this case, however, it is more illusionistic, as the gradual changes in the shapes of the grey forms seem to suggest two abutting cylinders receding,into the picture frame. The fame of such works not only epitomised the way in which art reached a wider audience in the 1960s, but also influenced fashion and a wider visual culture.