336 CARNEGIE INSTITUTION OF WASHINGTON. 



The fifth section begins the development of displacement inter- 

 ferometry in connection with the older Jamin-Mach interferometer, 

 an instrument which has certain pecuUar advantages and is in a meas- 

 ure complementary to the Michelson interferometer. The work was 

 undertaken in connection with the micromeasurement of the difference 

 of heights of communicating columns of liquids. 



The chief result of the paper is the detection of the achromatic inter- 

 ferences (so called for convenience), interferences which are ulti- 

 mately colors of thin plates seen at obUque incidence. But with the 

 new interferometer and obtained with white light, they are pecuharly 

 straight and vivid and resemble a narrow group of sharp FresnelUan 

 fringes with the central member nearly 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 so mobile in connection with micro- 

 metric displacement that in general it would be almost hopeless to find 

 them but for the fact that they coincide in adjustment with the cen- 

 tered elhpses or hyperbolae of the spectrum fringes of the displacement 

 interferometer. The fine, white slit-image which is dispersed to pro- 

 duce the latter carries the achromatic fringes when the sht is indefi- 

 nitely broadened or removed. The paper shows a curious method for 

 the measurement of vertical displacements, possibly available for the 

 detection of ether drag, which though just insufficient in connection 

 with the spectrum fringes would be promising in connection with the 

 achromatic fringes. Finally, the paper contains some work similar to 

 the old experiments of Fizeau on the periodic evanescence of fringes 

 due to the sodium fines. 



The peculiar adaptabiUty of the new interferometer to the measure- 

 ment of small angles, either in a horizontal or a vertical plane, is devel- 

 oped in the final section. The ratio of the displacement of fringes and 

 the angular displacement to be measured may be made enormously'' 

 large, and the paper shows cases in which, with strong luminous fringes, 

 the angle to be measured is magnified 500 times. Moreover, this is by 

 no means a limiting performance. Again, while angles as small as a few 

 tenths second or less are measured, angles as large as several degrees 

 come naturally within the scope of the method. Similar remarks may 

 be made with respect to the ratio of angular displacement and microm- 

 eter displacement. Given, therefore, an apparatus which measures 

 very small angles without constraint or forced approximations, the 

 measurement of long distances is the next result in order; for it is 

 merely necessary to place the angle to be measured at the apex of the 

 distance triangle on the length b of the ray parallelogram as a base. 

 This may be done in a variety of ways, some of which are shown in 

 part 2 of publication 249. 



