4 PREFACE. 



by the form of the vibration ellipses obtained, whether modified by resistance, 

 inductance, or capacity. So sensitive an apparatus naturally catches all the 

 quivering stray magnetic fields in the room; but here again any such effect, 

 which might at first sight seem to be fatal, may be compensated by the 

 primary solenoid (for instance) almost as easily as the needle of an astatic 

 galvanometer. Indeed, in the absence of current (secondary), the needle may 

 be given any reasonable amplitude or phase. It is shown, furthermore, that 

 the persistence of the symmetrical ellipse, with its axes respectively parallel 

 to the directions of vibration, is a strikingly accurate criterion of resonance. 



Chapter VII shows that a slight but essential modification of a form of 

 interferometer used by Michelson and Morley, makes this apparatus virtually 

 self-adjusting, while satisfying many of the requirements of displacement 

 interferometry. This is a very great convenience when many separate adapta- 

 tions of apparatus to the interferometer have to be made successively; for 

 the wearisome search for fringes is thus reduced to a minimum. It is even 

 possible to put a part of one of the mirrors of the interferometer on a microm- 

 eter screw for direct measurement, though the instrument is then no longer 

 quite self-adjusting. The endeavor to use this device for finding the refrac- 

 tion of solid media apart from form did not, however, furnish results of prac- 

 tical value. On the other hand, a possible design of this kind for measuring 

 the Fresnel coefficient is tested with a promising outcome in Chapter VIII. 



An interesting class of interferences obtained by superposing the fringes 

 resulting from dispersion on identical fringes resulting from the inclination of 

 rays, is discussed in Chapter IX. It is possible in this way to obtain sharp 

 spectrum fringes in the very luminous spectrum of an indefinitely wide slit and 

 to specify the angular orientation of the spectro-telescope on its axis ; for the 

 fringes, if small, jump out of an unbroken spectrum band suddenly, when 

 a definite angle is reached. Both of these possibilities are of practical value. 



A number of results incidental to the preceding work are collected in 

 Chapter X. Evidences of continuous micrometric convection currents within 

 liquids, obtained from the shadows of motes in a highly dispersed spectrum, 

 the satellites of the achromatic fringes already referred to in the preceding 

 report, peculiarly brilliant phenomena obtainable in connection with Her- 

 schel's fringes, and other subjects are here treated. 



Finally, in Chapters XI and XII, I have returned to certain gravitational 

 experiments begun in the last report. The former, in which the deviations 

 of the horizontal pendulum are read off by the displacement of achromatic 

 fringes, is very definite in its evidence of the effect of temperature distributions 

 within the supporting pier. Chapter XII is a continuation of the endeavor 

 to follow the actual motion of a gravitation needle, under periodic gravita- 

 tional attraction, with a view to deducing conclusions from that motion. 

 The apparatus ultimately met with serious accident in the endeavor to exhaust 

 it ; but though the experiments have not been concluded, the progress made 



is encouraging. 



CARL BARUS. 

 BROWN UNIVERSITY, July, 1919. 



