A PHOTOGRAPH showing an enlargement of the green mercury line in the third-order spectrum obtained by means of an aluminized diffraction grating with 30,000 lines to the inch is submitted by Prof. R. W. Wood. Gratings with 15,000 lines to the inch give high concentration of light in the first-order spectrum and have been used with very good results at Mt. Wilson Observatory. Replicas of such gratings made by flowing on them a nitrocellulose solution have been successfully used by the Lick and Harvard Observatories for studying the extreme red and infra-red spectra of the stars, and for obtaining the distribution of the different gases in nebulae. Prof. Wood hopes to prepare both plane and concave gratings by the use of plastics and a moulding process.