The deflection of light passing close to a mass distribution was already mentioned by Isaac Newton in the 18th century. However it was only after the formulation of general relativity by Albert Einstein that the behavior of light in a gravitational field could be studied on a firmer theoretical ground. The first calculations related to gravitational lensing were found in some unpublished notes from 1912, where Albert Einstein considered a star perfectly aligned with a foreground mass, concluding that the source should be imaged as a ring (now called the Einstein ring) around the deflecting mass. If the alignment is not perfect, two images of the background star would be visible on either side of the lens. Einstein (1936) computed the typical separation of such images and concluded that the separation would be so small (a few milli-arcseconds) that, as he wrote, “there is no great chance of observing this phenomenon”.