6 PREFACE. 



Spectra obtained with two, or at times even with one grating, are often 

 annoyingly furrowed with large transverse fringes. These are investigated in 

 Chapter VI, and referred to diffractions resulting from residual errors in the 

 rulings. In Chapter VII, finally, several examples of new methods of investi- 

 gation are given. They show the important bearing of the diffraction at 

 the slit of the collimator on all these experiments. The cleavage of a field 

 of diffracted rays as an essential preliminary is here put in direct evidence. 



In Chapters VIII to XIII I have returned to my older experiments with 

 the displacement interferometer. The subjects adduced, like the dispersion 

 of air at low and high temperatures, the adiabatic expansion of air, etc., are 

 pursued less with the object of reaching results of precision than of testing 

 the limits of the displacement method and developing it. 



My thanks are due to Miss Abbie L. Caldwell for very efficient assistance 

 in preparing the manuscript and drawings for the press. 



CARL BARUS. 

 BROWN UNIVERSITY, Providence, Rhode Island. 



