abstracts: physics 585 



curve characteristic of a pyroelectric conductor. The strips are sen- 

 sitive to small alternating currents, which increase the resistance 

 enormously, while small direct currents have the opposite effect. The 

 specific resistance has been measured and experiments made on the 

 electrochemical decomposition. G. W. V. 



OPTICS. — A?i investigation of the axial aberrations of lenses. E. D. 

 Tillyer and H. I. Schultz. Bureau of Standards Scientific 

 Paper No. 311 (Bull. Bur. Stds. 14: 341-369). 1917. 

 The errors which affect the definition of a lens are discussed, and 

 methods of graphically representing the central errors described. The 

 condition for freedom from coma near the axis is arrived at. The 

 relative importance of the errors in different types of lenses is dis- 

 cussed. Hartman's method is extended, permitting one set of measure- 

 ments to give all the important central errors — spherical aberration, 

 zonal variation of equivalent focal length, and axial and oblique achrom- 

 atism. The apparatus and procedure are described, and the accuracy of 

 the adjustments and of the measurements discussed. The method is ap- 

 plicable to all systems of relatively short focus and large aperture, such as 

 photographic lenses, projection lenses, and telescope objectives, and also 

 to complete optical systems. The results of the method as applied 

 to a complete telescope are discussed, and shown to be independent 

 of the accommodation of the observer. Seventeen sets of curves are 

 given for as many different lenses, and an illustrative discussion of 

 one set of curves, together with a general description of the types of 

 lenses represented by each group of curves is included. E. D. T. 



SPECTROSCOPY. — Wave-length measurements in spectra from 5900 A 



to 9600 A. W. F. Meggers. Bureau of Standards Scientific 



Paper No. 309 (Bull. Bur. Stds. 14: 317-329). 1917. 



Plates stained in a mixture of dicyanin, water, alcohol, and ammonia 



were used to photograph the arc spectra of twenty of the chemical 



elements, including the alkali metals, the alkaline earths, and elements 



commonly found in iron as impurities; a concave grating of 640 cm. 



radius, mounted in parallel light was used. This spectrograph gives 



o 



a dispersion of about 10A per millimeter in the first order. Exposures 

 of thirty minutes sufficed to record waves longer than 9000 A. In 

 this paper the wave-lengths in International Angstroms are given for 

 the arc spectra of the following elements: lithium, sodium, potassium, 

 rubidium, caesium, copper, calcium, strontium, barium, and magnesium. 



