PHYSICS. 311 



PHYSICS. 



Barus, Carl, Brown University, Providence, Rhode Island. Grant No. 

 857, allotted December 13, 1912. Study of the application of dis- 

 placement inter ferometry. (For previous reports see Year Books 

 Nos. 4, 5, 7-11.) $500 



Professor Barus is at present engaged in collating the data of a 

 report containing a variety of investigations made at widely different 

 times, but in all of which the displacement interferometer was used 

 as a basis of measurement. It is thus a continuation of the work in 

 Carnegie Institution of Washington publication No. 149. 



In the first chapter, the distance apart of the doublet interference 

 patterns, obtained with plates of doubly-refracting crystals, is used 

 for computing the corresponding ordinary and extraordinary indices 

 of refraction of these crystals in the case of light travehng through 

 them in different directions. As quartz gives particularly beautiful 

 and well-placed images, measurements were made in greatest number 

 by means of it, though other bodies are also studied. It is an essen- 

 tial feature of the method that the center of ellipses is never lost, but 

 may be brought back into coincidence with the fiducial sodium line 

 of the spectrum by the proportionate displacement of the mirror on 

 the micrometer. Hence plates of glass or crystal of any thickness, 

 or indeed, columns of any length, are available for measurement. 

 These wide limits of application are peculiarly favorable to certain 

 accurate measurements. For instance, if a glass column 1 meter 

 long is inserted, and the micrometer reads to but 0.5 X 10^ centimeter, 

 IX — \ (where ^ is the index of refraction) would be measurable to one 

 part in a million. The method should, therefore, be useful to answer 

 certain questions in refraction. 



The second chapter contains a number of miscellaneous measure- 

 ments made on the displacement interferometer, or bearing upon it. 

 Thus the ease with which it lends itself to the comparison of screws 

 of any length is illustrated by many trials; a simple type of long 

 screw micrometer is designed and investigated; a method for the 

 measurement of very small increments of angle, consisting essen- 

 tially in mounting two small mirrors in parallel and symmetrically 

 to the axis, is tested; advantages in obtaining elliptic interferences 

 when the reciprocating opaque mirrors are concave are discussed, 

 etc. Furthermore, the availability of the interferometer in measuring 

 certain acoustic displacements, such as occur in the Dvorak and 

 Mayer experiments and telephone plates, is investigated. It is then 

 shown that an induction balance of a peculiar kind may be con- 

 structed by replacing the opaque mirrors of the interferometer by 

 mirrors attached to the synchronized and reciprocating plates of 



