We employ VLTI GRAVITY to resolve , for the first time , the two images generated by a gravitational microlens .
The measurements of the image separation \Delta \theta _ { - , + } = 3.78 \pm 0.05 mas , and hence the Einstein radius \theta _ { E } = 1.87 \pm 0.03 mas , are precise .
This demonstrates the robustness of the method , provided that the source is bright enough for GRAVITY ( K \lesssim 10.5 ) and the image separation is of order or larger than the fringe spacing .
When \theta _ { E } is combined with a measurement of the “ microlens parallax ” \pi _ { E } , the two will together yield the lens mass and lens-source relative parallax and proper motion .
Because the source parallax and proper motion are well measured by Gaia , this means that the lens characteristics will be fully determined , whether or not it proves to be luminous .
This method can be a powerful probe of dark , isolated objects , which are otherwise quite difficult to identify , much less characterize .
Our measurement contradicts Einstein ’ s ( 1936 ) prediction that “ the luminous circle [ i.e. , microlensed image ] can not be distinguished ” from a star .