Besides a good light source, the other element needed to get a clear image with a direct ophthalmoscope is a correcting lens. The correcting lens was needed to get the best image based on the refraction of the patients pupil. He Helmholtz ophthalmoscope had a single lens, so correction was not possible. Andrew Stanford Morton, of London, solved the problem by adding a dial with additional lenses into the rotating mirror design of Lindsay. Once the lens was selected, it could be slid into place behind the viewing port.