Chemistry and Physics 47 



character make up the first eleven chapters of the book. The remaining six chapters 

 are devoted to new investigations with the displacement interferometer, such as the 

 use of curvilinear compensators, the dispersion of air, etc. 



No. 249. (Part II.) The Interferometry of Reversed and Non-reversed Spectra. 

 Octavo, 146 pages, 97 figs. "Published 1917. Price $1.50. 



The author pursues the work on the interferences of reversed and non- 

 reversed spectra, begun in the last report (Carnegie Inst. Wash. Pub. No. 249, 1916), 

 in a variety of promising directions, such as the original investigation suggested. 

 The apparatus has been extensively modified, so as to admit of measurements 

 relating to individual fringes. A large part of Chapter I is devoted to the treat- 

 ment of prismatic methods, developed with the additional purpose of securing 

 a greater intensity of light and of showing interference of spectra from the same 

 source, but of different lengths (dispersion) between red and violet. A type of 

 fringes is detected which depends merely on the grating space and is indepen- 

 dent of wave-length. The limits of micrometer displacement within which 

 fringes of any kind are discernible is shown to be a necessary result of disper- 

 sion. The direct interference of divergent rays obtained from polarizing media 

 is exhibited. In Chapter II the interferences of inverted spectra are given 

 greater prominence. Spectra half reversed and spectra both reversed and inverted 

 are treated successfully. The third and fourth chapters are incidental applica- 

 tions of the displacement interferometer and contain experiments on the expan- 

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

 the refraction of glass, irrespective of form. Chapter V begins the development 

 of displacement interferometry in connection with the older Jamin-Mach inter- 

 ferometer, an instrument which has certain peculiar advantages and is in a mea- 

 sure complementary to the Michelson interferometer. The chief result of Chapter 

 V is the detection of the achromatic interferences, as they are called for con- 

 venience interferences which are ultimately colors of thin plates seen at oblique 

 incidence ; but with the new interferometer, and obtained with white light, they 

 are peculiarly straight and vivid and resemble a narrow group of sharp Fres- 

 nellian fringes with the central member in black and white. They are capable of 

 indefinite magnification and their displacement equivalent is a fraction of a mean 

 wave-length per fringe. Notwithstanding their strength and clearness, they are 

 strikingly mobile in connection with micrometric displacement. The peculiar 

 adaptability of the new interferometer to the measurement of small angles, either 

 in a horizontal or a vertical plane, is developed in the final chapter. The paper 

 shows cases in which, with strong luminous fringes, the angle to be measured 

 is magnified 500 times and this is by no means a limiting performance. 



No. 249. (Part III.) Displacement Interferometry by the aid of the Achromatic 

 Fringes. Octavo, 100 pages, 71 figs. Published 1919. Price $1.50. 



Part III is chiefly devoted to the investigation of methods of research in which 

 displacement interferometry conducted by the aid of the achromatic fringes dis- 

 cussed in the preceding report, gives promise of fruitful applications. In Chapter 



I the method of measuring small angles is put to a practical test. The general 

 theory of the subject is developed at some length and a variety of interferometer 

 devices, with mirror, ocular, and collimator micrometers are instanced. Chapter 



II is devoted to spectrum fringes differing in their manner of production, the 

 endeavor being made to obtain interferences from distant slender luminous objects 

 without the aid of a slit. Chapter III treats of the endeavor to obtain the elastic 

 constants of small bodies. Chapter IV contains applications of the rectangular 

 interferometer using achromatic fringes to geophysical problems. A method for 

 the determination of the Newtonian constant is worked out; the same inter- 

 ferometer is associated with the horizontal pendulum for the detection of small 

 changes in the inclination of the earth's surface. The final chapter details cor- 

 responding methods for the interferometry of vibrating systems. 



