GEOPHYSICAL LABORATORY. 169 



arising from the bulky form in which it comes from the producer, its hetero- 

 geneous character, and the arithmetical or psychological limitations peculiar 

 to each method of distribution. The bearing of these various factors on ex- 

 isting methods of distribution, both to producers of information and to the 

 general pubUc, is touched upon, and desirable or probable future develop- 

 ments are briefly discussed. 



(409) Note on the measurement of the density of minerals. L. H. Adams. Am. Mineral- 



ogist, 6, 11-12 (1921). 



The striking variations shown in different determinations of even the most 

 simple and definite minerals indicate that there is room for improvement in 

 the technic of density-measurement. In this note attention is called to the 

 advantages of the "flat-top" pycnometer for measuring the density (or specific 

 gravity) of minerals and other granular or powdered soUds. With this 

 pycnometer an accuracy of about 0.0001 can be obtained. 



(410) War-time production of optical munitions. Fred. E. Wrie;ht. Army Ordnance, 1, 



247-251 (1921). (Papers on Optical Glass, No. 33.) 



In this paper a general statement is given of the development of the optical 

 glass and instrument situation during the recent war. Some of the difficulties 

 which arose and the measures taken to overcome them are described briefly. 

 The records show that the methods for the manufacture of optical glass had 

 first to be developed and placed on a routine basis ; factory f acihties had greatly 

 to be increased; raw materials of adequate purity had to be obtained and 

 transported ; a personnel competent to handle the different phases of the prob- 

 lems had to be organized and trained to the several tasks. Methods for the 

 adequate inspection of optical glass had to be adopted and a force of inspectors 

 trained in their use. The manufacturing capacity of the country for optical 

 instruments had to be increased greatly; new operators had to be trained, 

 and the entire optical industry organized and coordinated. The roles played 

 in this connection by the manufacturers, the Geophysical Laboratory, the 

 War Industries Board, and the War and Navy Departments are noted briefly. 

 The significance of the optical industry in war time is emphasized, and means 

 are suggested for the development during peace-time of a skeleton organiza- 

 tion of optical engineers who, in the case of an emergency, might render effec- 

 tive aid in building up the optical industry to meet the needs of the field 

 forces. 



(411) The angular deflections produced on transmitted light rays by slightly incorrect 



interfacial angles of reflection prisms. Fred. E. Wright. J. Opt. Soc. Amer., 

 5, 193-204 (1921). (Papers on Optical Glass, No. 34.) 



The interfacial angles of a reflecting prism of any given type may be meas- 

 ured directly on a goniometer or a spectrometer, or by means of dehcate gages 

 or specially mounted test plates such as the optical square. They may also 

 be measured by the deflections produced by the prism on transmitted rays of 

 light. A prism of the prescribed shape deflects the transmitted rays along 

 a prescribed path. A prism of slightly incorrect interfacial prism angles de- 

 flects transmitted Ught rays so that these emerge along a direction sUghtly 

 different from that prescribed ; from this deviation it is possible to determine 

 the degree of exactness of the interfacial prism angles. Methods of test 

 based on this principle enable the observer at the same time to draw conclu- 

 sions regarding the quality of glass in the prism and the degree of flatness of 

 the prism faces; in short, to judge of the fitness of the prism as a component 

 of any given optical system. In the present paper prisms of different types 

 are considered, and sets of curves are shown in diagrams illustrating the devia- 



