A measure of the ability of an optical instrument to form separable images of close objects or to separate close wavelengths of radiation. The chromatic resolving power for any spectroscopic instrument is equal to λ/δλ‎, where δλ‎ is the difference in wavelength of two equally strong spectral lines that can barely be separated by the instrument and λ‎ is the average wavelength of these two lines. For a telescope forming images of stars the angular resolving power is the smallest angular separation of the images; the linear resolving power is the linear separation of the images in the focal plane. In a telescope forming images of two stars, as a result of diffraction by the lens aperture each image consists of a bright central blob surrounded by light and dark rings. According to the Rayleigh criterion for resolution, the central ring of one image should fall on the first dark ring of the other. The angular resolving power in radians is then 1.22λ‎/d, where d is the diameter of the objective lens in centimetres and λ‎ is the wavelength of the light (usually taken as 560 nanometres). For microscopes, the resolving power is usually taken as the minimum distance between two points that can be separated. In both cases, the smaller the resolving power, the better the resolution; to avoid this apparent paradox the resolving power is now sometimes taken as the reciprocals of the quantities stated above.