S e p t . , I 92 t . 1 ~ { E C H A N I C A L ~ ' R E Q U E N C Y - ~ I E T E R . 359

accordance with the frequency to be measured. In the instrument described the recommended weights are as follows :

Frequency Best weight Cycles per second Grammes

250 to 500 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 900

500 to 750 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I35O

750 to i25o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2700

125o to I75O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4050

In the calibration of any particular instrument of this type, the best weights to use can readily be determined experimentally.

The authors desire to acknowledge their indebtedness, not only to Prof . L. V. King for the original idea of the instrument, but also to Prof . V. Bush, and to Prof . F. S. Dellenbaugh of our department, for help and valuable suggestions.

Death of Lippmann . - - J . F. Gabriel Lippmann, the inventor of the Lippmann process of color photography, died in July last. He was bor~ in 1845, in Luxembourg, of Alsatian-Lorraine parents, but went early to' France, where he was educated and where his scientific work was all carried out. He received many honors, the greatest being the award of a Nobel prize in physics in 1918. His first scien- tific contribution was an essay on electro-capillarity, submitted as a thesis for the degree of Doctor of Sciences. In 1886 he was elected a member of the Academy of Sciences and numerous other appoint- ments of trust and research were later given to him. The first notice of his remarkable procedure for obtaining color photographs by the interference method was made to the Academy in February, 1891 , when he exhibited a photograph of a spectrum in its true colors. The paper appears on C.r., 1891, v. 112, 274. In 1892 the French Photographic Society awarded him the Jannsen medal, and in 1897, the " Progress " medal was bestowed by the Royal Society of Lon- don. Lippmann's interest in photography continued through life and among his later contributions 'is one on the use of color screens in orthochromatic work.

The Lippmann process is simple in general principle but has never come into active use. It was not patented, and the inventor is said to have expressed regret that he did not at once take out patents, as he thought possibly this might have encouraged commercial ex- ploitation. A recent issue of La Revue Franfaise de Photographic gives the story of his life in some detail, and a brief account of the particular procedure upon which his fame principally rests.

H . L .

Dry Method for the Preparation of Lead Arsenate. O . W . BROWN, C. R. VORIS, and C. O. HENKE, of Indiana Univers i ty (Jour. Ind. and Eng. Chem., 1921 , xiii, 531-533), have devised a