They found what appeared to be two identical quasars - the first gravitational lens. Since then many other lenses have been identified. It seems that about one distant radio source in 500 is split into multiple images because of gravitational lensing by a foreground galaxy. The separation of the images produced by gravitational lenses is typically one arcsecond but in order to make sure that the source is a lens, a resolution much better than that is required. The round blobs are all supernova remnants.
Gravitational lenses have exciting cosmological implications.
Their bending of radio waves offers one of the very few ways of measuring the mass of a galaxy. Moreover, with a large sample of lenses we can test the predictions of cosmological models and hence study the geometry of the universe. By measuring the brightness variations in different parts of the image, it should be possible to estimate the distance to the galaxy and hence the rate of expansion and size of the universe. VLBI techniques come into their own for high-resolution studies of individual lensed systems which are important if the mass of the lensing galaxies is to be found. Photographs show extensive filamentary streamers extending away from the centre.