238 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 10 



PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



PHILOSOPHICAL SOCIETY 



843d MEETING 



The 843d meeting of the Philosophical Society of Washington was held 

 in the Cosmos Club, December 18, 1920, with President Faris in the chair, 

 and 52 persons present. 



The first paper was by R. W. G. WyckoFF, on The determination of the 

 structure of crystals, and was illustrated with lantern slides. 



An outline of the development of the methods thus far used serves to show 

 the point of view from which the studies of the arrangement of the atoms 

 in crystals have been carried out. The essential steps in this development 

 are the experiments of Laue and of the Braggs, the determination of the ar- 

 rangement of the atoms in some one crystal and the consequent measurement 

 of the absolute wave lengths of X-rays. In nearly all of the structures 

 which have been studied a procedure based upon the point of view of these 

 experiments has been followed. This procedure consists in getting a lim- 

 ited amount of experimental data with the aid of one of the three existing 

 methods of obtaining diffraction effects — ^the Laue method, the spectro- 

 meter method or its modification, and the method of powders. Bearing 

 these experimental facts in mind, the analyst has tried to imagine some 

 arrangement of atoms which will explain them. If he has succeeded in 

 devising such a grouping, it is considered to be the structure of the crystal. 

 This method of procedure is both cumbersome and haphazard and there is 

 no means of knowing whether many other ways of arranging the atoms of 

 the crystal under examination do not exist, all equally capable of explaining 

 the data. 



This method is to be contrasted with the general method for studying 

 the structure of crystals which can be built around the theory of space 

 groups. The results of this geometrical theory can be given an analytical 

 representation which states in terms of suitable coordinates all of the posi- 

 tions in space that atoms in crystals can occupy. With its aid, and knowing 

 the crystallographic symmetry, it is thus possible to write down in the case 

 of any particular crystal, independently of any X-ray experimentation, all 

 of the ways in which the atoms can be arranged. Suitable calculation will 

 give the kind of dift'raction effects to be anticipated from each of these pos- 

 sible arrangements of atoms. The experimenter may then obtain, by which- 

 ever of the various methods will yield the desired information most readily, 

 those data which the calculations show to be necessary to distinguish be- 

 tween the possible atomic groupings. 



The second paper, by I. G. Priest and Mabel K. FrEhafer, on The optical 

 basis of Bitiinger's camouflage paintings, was presented by Mr. Priest, and 

 was illustrated with slides and with examples and copies of the paintings. 



Viewed in daylight, or any illumination approximating to this, these paint- 

 ings appear no different from other paintings of the same kind ; but if viewed 

 through a red glass, or in red light, the aspect of the picture is changed 

 materially. A picture of the sea, with a wave breaking, shows under the red 

 illumination, a mermaid rising from the wave. A summer scene in which 



