364 CARNEGIE INSTITUTION OF WASHINGTON. 



primary. The sensitiveness may be increased upwards 100 fold, how- 

 ever, so that 10-^ ampere per fringe is still measurable, by placing a 

 similar instrument on the displacement interferometer adjusted for 

 achromatic fringes. The reading in such a case must be made with 

 a vibration telescope, synchronized with the alternating current in the 

 primary and with the objective vibrating normally to the displacement 

 of fringes. The measurement is thus somewhat awkward and consists 

 in determining the range of the fringe ellipses parallel to the direction 

 of vibration of fringes. On the other hand, both the amplitude and the 

 phase of the induced current, whether modified by resistance, induc- 

 tance, or capacity, is given by the form of vibration ellipses obtained. 



A slight but essential modification of a form of interferometer used 

 by Michelson and Morley makes the apparatus virtually self-adjusting, 

 yet satisfying many requirements in displacement interferometry. 

 This is a great convenience when many separate adaptations of appara- 

 tus to the interferometer have to be successively made. It is even 

 possible to put a part of one of the mirrors on a micrometer-screw for 

 direct measurement. The endeavor to use this device for finding the 

 refraction of solid media did not, however, lead to results of practical 

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

 the Fresnel coefficient is being tested with a promising outcome. 



An interesting class of interferences is obtained by the superposition 

 of fringes due to dispersion on identical fringes due to the inclination 

 of rays. It is possible in this way to obtain sharp spectrum fringes in 

 the very luminous spectrum of an indefinitely wide slit and to deter- 

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

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

 when a definite angle is reached. A number of results incidental to the 

 preceding work are shown. Evidences of continuous micrometric con- 

 vection currents within liquids, obtained from the shadows of motes 

 in a highly dispersed spectrum; the satelhtes of the achromatic fringes 

 already referred to in a preceding report; peculiarly brilliant phe- 

 nomena obtainable in connection with Herschel's fringes; and other 

 subjects, are here treated. 



The gravitational experiments begun in the last report have been 

 continued. 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 pier. The author is inclining to the conviction that measure- 

 ments of the gravitational attraction of two bodies made daily, for a 

 period of years, might not be unproductive of results. To do this 

 effectively, however, a full analysis of the thermal and other radiation 

 discrepancies must first be available, and work with this end in view is 

 being actively pursued. 



