Thus common-tone modulations are convenient for modulation by diatonic or chromatic third. This term covers both chromatic and diatonic transpositions as follows. A chromatic and a diatonic note, or two chromatic notes, create chromatic intervals. The distinction between diatonic and chromatic intervals may be also sensitive to context. Refractors suffer from residual chromatic and spherical aberration. However, strong eyeglass prescriptions create distortions such as prismatic movement and chromatic aberration. Additionally, the label "chromatic" or "diatonic" for an interval may be sensitive to context. The book contains detailed descriptions of phenomena such as coloured shadows, refraction, and chromatic aberration. Flickers were created through a mixture of motion blur, chromatic aberration, and focus effects. In 1733, he succeeded in constructing telescope lenses which exhibited much reduced chromatic aberration. He concluded that light could not be refracted through a lens without causing chromatic aberrations. Data rates are limited by polarization mode dispersion and chromatic dispersion. Most contemporary hurdy-gurdies have 24 keys that cover a range of two chromatic octaves. These keys provided for simple chromatic fingerings. Contrabass strings are arranged in a descending chromatic scale.