A device embodying Robertson's principle of movable strips is described for performing two‐dimensional Fourier summations of the type&Sgr;H&Sgr;KF(hk)[cos2&pgr;(hX+kY)]without first rearranging them in the form&Sgr;H&Sgr;KF(hk)[cos2&pgr;hX·cos2&pgr;kY−sin2&pgr;hX·sin2&pgr;kY]. The same strips can be used to evaluate sine summations and some three‐dimensional summations directly. The electron density can be readily calculated at selected individual points in the projection. Summations can also be completed by means of masks. The operation of the device, which can accomodate values ofF(hk) = ± (1 to 99),h= −15 to +15,k=0 to 15 forX, Y= 1/36 of the repeat distance, is described, and illustrated by an example.