In 1931, Urey set out to find it. Urey and George Murphy calculated from the Balmer series that the heavy isotope should have lines redshifted by . Urey had access to a grating spectrograph, a sensitive device that had been recently installed at Columbia and was capable of resolving the Balmer series. With a resolution of 1 Å per millimetre, the machine should have produced a difference of about 1 millimetre. However, since only one atom in 4,500 was heavy, the line on the spectrograph was very faint. Urey therefore decided to delay publishing their results until he had more conclusive evidence that it was heavy hydrogen.