iv PREFACE. 



by lengthening the column of glass, is not fulfilled. The ellipses become pro- 

 portionately more sluggish in their motion as the path difference is increased. 



In Chapter IV a number of incidental experiments, on allied subjects, have 

 been grouped together. In the first paper the possible bearing of certain disk 

 colors of circular gratings on the somewhat similar phenomenon in coronas 

 is discussed. The second paper deals with the performance of the easily 

 available film grating to replace the ruled-glass grating, for purposes of dis- 

 placement interferometry, from a practical standpoint. With the same end 

 in view the third paper considers the use of the Nernst filament as an available 

 illuminator, in the absence of the arc lamp or sunlight. In conclusion, an 

 interesting case of regular reflection and refraction of scattered light, bearing 

 on the X-ray phenomena recently discovered by Professor Bragg, is treated 

 in the fourth paper. 



In Chapter V, finally, following the suggestive experiments made in an 

 earlier report, the displacement interferometer is directly applied to the 

 quadrant electrometer. In the several hundreds of adjustments made no 

 serious difficulty was encountered in the optical parts of the experiments, 

 and that was the question chiefly at issue. The sensitiveness obtained in 

 this way should have been of the order of a millionth of a volt per vanishing 

 interference ring; but owing to the uninterrupted commotions surrounding 

 the laboratory already referred to, possibly also to difficulties residing in the 

 electrometer, this limit could not be reached. The experiments, therefore, 

 largely explore the available scope of the method. 



My thanks are due to Mrs. D. T. Knight and to Miss R. R. Snow for 

 efficient assistance in connection with the preparation of the papers for the 

 press. 



CARL BARUS. 

 BROWN UNIVERSITY, September 25, 



