PHYSICS — BARUS. 23 1 



In the direct method, a mirror immediately behind the grating returns the 

 reflected-diffracted and diffracted-reflected component rays, to be superim- 

 posed for interference, producing a series of phenomena which in addition to 

 their great beauty are eminently useful. In fact, the interferometer so con- 

 structed needs but ordinary plate-glass and replica-gratings. It gives equi- 

 distant fringes, rigorously straight, and their distance apart and inclination 

 are thus measurable by ocular micrometry. The fringes are duplex in char- 

 acter and an adjustment may be made whereby ten small fringes occupy the 

 same space in the field as one large fringe, so that sudden expansions within 

 the limits of the large fringe (as, for instance, in magneto-striction) are 

 determinable. This has not been feasible heretofore. Lengths and small 

 angles (seconds of arc) are thus both made subject to micrometric measure- 

 ment. Finally, the interferences are very easily produced and are strong with 

 white light, while the spectrum-line used may be kept in the field as a station- 

 ary landmark. The limiting sensitiveness is half the wave-length of light. 



Professor Barus, assisted by Mr. M. Barus, has worked out the theory, 

 has devised advantageous instrumental equipment, and has made a number 

 of incidental applications to test the apparatus. 



The range of measurement of such an instrument is necessarily limited to 

 about 1 cm. and the component rays are not separated. To increase the range 

 indefinitely and to separate the component rays, let the grating replace the 

 symmetrically oblique transparent mirror of the Michelson adjustment, for 

 instance. In this way transmitted-diffracted and reflected-diffracted spectra, 

 or two corresponding diffracted spectra returned by the opaque mirrors, may 

 be brought to interference. In the former, but particularly in the latter case, 

 the author has been strikingly successful. The interference pattern, how- 

 ever, is now of the ring type, extending throughout the whole spectrum from 

 red to violet, with the fixed spectrum-lines simultaneously in view. These 

 rings closely resemble confocal ellipses ; their centers have the same position 

 in all orders of spectra, but the major axes of the ellipses are liable to be 

 vertical in the first and horizontal in all the higher orders of spectra. Again, 

 there is an opportunity for coarse and fine measurement, inasmuch as the 

 rings have the usual sensitive radial motion as the virtual air-space increases 

 or decreases, while the centers simultaneously drift as a whole across the 

 fixed lines of the spectrum, from the red to the violet end. Drift and radial 

 motion may be regulated in any ratio, and this is one advantage of the method. 



Professor Barus shows that three groups, each comprising a variety of 

 interferences, are possible, and has worked out the theory of the phenomenon 

 and the instrumental conditions for efficient practical work. Transparent sil- 

 vered surfaces are superfluous, as the ellipses are sufficiently strong not to 

 need accessory treatment. Considerable width of spectrum-slit is also ad- 

 missible. Finally, the ellipses may be made of any size and the sensitiveness 

 of their lateral motion may be regulated to any degree by the aid of a com- 

 pensator. In this adjustment the drift may be made even more delicate than 

 the radial motion, thus constituting an entirely new method of interferometry. 



