The so-called gravitational lens effect, previously worked out by Tillhov in 1937, is derived in a simple manner. The effect is caused by the gravitational deflection of light from a star S in the gravitational field of another star B, and occurs when S lies far behind B, but close to the line of sight through B. It turns out that a considerable increase in the apparent luminosity of S is possible. A method is given to determine the mass of a star which acts as a gravitational lens. The possibillty of observing the effect is discussed.