CONTENTS 
Paij;e 
Mathematics. — Projective and Affine Geometry of Paths . By 0.^ Vehlen 347 
Physiology. — A Direct Method of Testing Color Vision in Lower 
Animals By W. F. Hamilton 350 
Physics. — Characteristics ^ of a Short Wave Oscillator at Very Low 
Pressures By L. L. Nettleton 353 
Physics.' — The Refraction of X-Rays in Calcite 
By Bergen Davis and H. M. Terrill 357 
Physics. — The Compressibility of Metals at High Pressures ... * 
By P. W. Bridgman 361 
vStatistics. — A Further Note on the Mathematical Theory of Popula- 
tion Growth By Raymond Pearl and Lowell J. Reed 365 
Index 36!! 
