PHYSICS. 405 



waves will be set up, as in the Lippman plates; these will impress an 

 inclined photographic plate with parallel lines, as in the experiment of 

 Wiener, and the only limit to the resolving power of the resulting 

 grating is that which depends on the degree of homogeneity of the hght 

 used. As some of the constituents of the radiations of mercury have 

 been shown to be capable of interfering with difference of path of over 

 1,000,000 waves, such a grating would have a resolving power exceeding 

 1,000,000. 



This investigation has had assistance from the Bache Fund of the 

 National Academy of Sciences, from the Carnegie Institution of Wash- 

 ington, and from the University of Chicago. 



In addition to the grateful acknowledgment to these institutions 

 I would add my high appreciation of the faithful services rendered by 

 Messrs. Julius Pearson and Fred Pearson. 



Nichols, E. L., Cornell University, Ithaca, New York. Systematic study of 

 the properties of matter through a wide range of temperatures. (For previous 

 reports see Year Books Nos. 4-13.) 



In continuing the study of the fluorescence absorption of the uranyl 

 salts we have had a large number of new salts prepared by Mr. D. T. 

 Wilber. All show fluorescence spectra of the same general type as those 

 of the salts previously studied. Wliile the investigation of the absorp- 

 tion of the new salts has not been completed, there is every indication 

 that the absorption spectra also consist of series of bands whose differ- 

 ent members are equally spaced on the frequency scale. The interval 

 between bands appears to be constant for any given salt and has nearly 

 the same value for all the uranyl compounds. The interval between 

 absorption bands, however, is considerably shorter than the interval 

 between the fluorescence bands of the same salt. 



In the case of a large number of salts Mr. Wilber has found it possi- 

 ble to prepare crystals that are large enough for individual study. 

 This has made possible an investigation of the influence of the plane of 

 polarization of the incident rays upon their absorption and exciting 

 power, and of the condition, as regards polarization, of the light emitted 

 during fluorescence. The double chloride of uranyl and ammonium 

 has shown itself particularly well suited to such an investigation, since 

 the bands of absorption and fluorescence are surprisingly sharp, even 

 at ordinary temperatures. The double salts of potassium, rubidium, 

 and caesium show the same characteristics. 



A prehminarj^ account of the results obtained has been pubUshed 

 in the Proceedings of the National Academy (E. L. Nichols and H. L. 

 Howes, vol. 1, p. 444, August 1915) and two more extended articles 

 will shortly appear in the Physical Review. It is found that (1) each 

 fluorescence band is plane-polarized; (2) when the carbon arc is used 

 for excitation, the intensity and polarization of each fluorescence band 



