PREFACE. 



The present volume contains applications of the displacement interfer- 

 ometer to subjects which suggested themselves from time to time. Unfor- 

 tunately it was not possible, in the laboratory of Brown University, which is 

 situated on a hill in the middle of a large city, to carry out any experiment 

 to its final degree of rigor. Quiet surroundings, a location free from tremor, 

 and irregular temperature variations would have been necessary. But the 

 development of methods of the kind in question was nevertheless quite feasi- 

 ble; and without attempting to push them to a limit, the range of application 

 could be fully investigated. 



Among the subjects selected for treatment was the horizontal pendulum. 

 In the first part of Chapter I certain available forms of the pendulum, with 

 and without a float, are considered and tested as to their discrepancies, 

 through long lapses of time, by a reflection method. Among the interesting 

 results obtained is the suggestion of an apparatus capable of measuring 

 changes of elongation to the amount of even less than 4Xio~ 10 of the total 

 length per vanishing interference ring. 



In the second part of the chapter the interferometer itself is used, a service- 

 able method of application worked out, and the range of application studied 

 through many months. With a relatively very wide scope (several seconds 

 of arc) there should be no difficulty, under proper surroundings, of measuring 

 changes of inclination as small as 3 X io~ 4 seconds of arc per interference ring, 

 and it is probable that one could reach smaller angles by modifying parts of 

 the pendulum. 



In Chapter II an attempt is made to use this interferential horizontal pen- 

 dulum for the measurement of the gravitational attraction of two parallel 

 disks. What was obtained, however, was a definite repulsion of the disks, 

 decreasing with their distance apart and appreciable even within 1.5 mm. of 

 this distance. As the method of measurement contemplates the viscosity of 

 the film of air between the disks, and as the effect of any natural charge or 

 potential would be insignificant in comparison with the forces observed, it is 

 probable that the repulsion in question is attributable to the molecular 

 atmospheres by which the disks are surrounded in air, supposing that such 

 atmospheres of gas increase in density as the surface of the disk is approached. 



Chapter III is introduced as a severe test on the interference equation 

 employed for the case of path differences resulting when glass columns as 

 much as 10 inches long are inserted in one of the component beams of the 

 displacement interferometer. It appears that the constants of any dispersion 

 formula may be obtained directly from these observations. The equations 

 for the relations of displacement and wave-length increments show, however, 

 that the anticipation of great precision in the determination of refraction, 



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