AbstractThe following paper contains an investigation on the resolving power of a telescope for double starts, in as much as it depends on a) the difference in magnitude between the components; b) the star's total brightness; and c) the magnifying power. As to a) and b), observations made with a 12 cm‐refractor led to formulas for deriving the limit of the resolving power of this instrument. As to c), it has been shown that only when observing bright stars with nearly equal components, the magnification has a distinct influence on the resolving power. Furthermore, it has been proved that the formulas found for the instrument in question are valid for any telescope. This theoretical result could be verified by an examination of the practical results mentioned by various double star‐observ