PHYSICS. 335 



PHYSICS. 



Barus, Carl, Brown University, Providence, Rhode Island. Continuation 

 of investigations in interferomelry. (For previous reports see Year Books 

 Nos. 4, 5, 7-15.) 



In a volume submitted to the Institution, Professor Barus pursues 

 the work on the interferences of reversed and non-reversed spectra, 

 begun in his last report (Carnegie Publication No. 249, 1916), in a 

 variety of promising directions, such as the original investigation 

 suggested. It will be remembered that the reversal (180°) here con- 

 templated takes place on a transverse line of the spectrum {%. e., a line 

 parallel to the Fraunhofer lines) , which thereby becomes a line of sym- 

 metry for the phenomena. Apparatus has been extensively modified 

 so as to admit of measurements relating to individual fringes. 



A large portion of the first section is devoted to the treatment of 

 prismatic methods, developed with the additional purpose of securing 

 intensity of light. A curious intermediate case between interferences 

 of reversed and non-reversed spectra is the pronounced interference 

 of spectra from the same source but of different lengths (dispersion) 

 between red and violet. The phenomena of crossed rays find a par- 

 allel occurrence in the present paper when similar gratings or prisms 

 disperse and subsequently recombine a beam of white light. A type 

 of fringes is detected which depend merely on the grating space and 

 is independent of wave-length. An interesting question as to the 

 limits of micrometer displacement within which fringes of any kind are 

 discernible (observations which were at first supposed to be due to the 

 degree of uniformity of interfering wave- trains) is eventually shown to 

 be a necessary result of dispersion. Finally, the direct interference of 

 divergent rays obtained from polarizing media is exhibited. 



In the second section the interferences of inverted spectra (a sub- 

 ject merely touched in the preceding volume) are given greater promi- 

 nence. In this case one of the two spectra from the same source is 

 inverted (180°) relatively to the other on a longitudinal axis {i. e., an 

 axis normal to the Fraunhofer lines), which thus becomes a line of 

 symmetry. In the development of the subject, spectra half-reversed 

 and spectra both reversed and inverted are treated successfully. In 

 the latter case the conditions of interference are fulfilled at but a 

 single point in the whole area of the field, yet the phenomenon is 

 pronounced. The hmits of micrometer displacement within which 

 interferences may be obtained are again determined. 



The third and fourth sections are incidental applications of the 

 displacement interferometer and contain experiments on the expansion 

 of metal tubes by internal pressure and on a promising method of 

 measuring the refraction of glass irrespective of form. 



