LONDON.- There is much that Peter Joseph and Carmen Herrera share in their lifelong dedication to abstraction. Both senior figures, their work avoids being heroic. Paintings often hinge on the juxtaposition of colour brought together in a way that creates a certain unity; titles are usually statements of material, visual fact. Joseph is best known for his two-colour paintings, those of the 1960s being boldly geometric, relying on primary colours and optical effects; for Herrera the more lyrical abstraction of her early paintings gave way to a focus on pure geometry. The precision of her work in black and white from the beginning of the 1950s, the simplicity of their geometric structure and austerity of her palette, prefigure and anticipate the optical, kinetic and hard-edge minimalism of 1960s New York with artists such as Ellsworth Kelly, Frank Stella and Kenneth Noland. However for Herrera she refutes the tag of Op Art feeling it too simplistic a description of what she was at