EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 41 



these circumstances. During an explosion, of course, the ellipses vanish, to 

 reappear, however, immediately afterward, sometimes with displacement, such, 

 for instance, as is indicated by the doubled parts of the curve. The use of the 

 water damper, moreover, which was necessary here, is objectionable, though 

 it has not, probably, introduced any marked error into the observed curve 

 (see doubled parts) . Finally, the use of a steel horizontal pendulum with its 

 plane in the magnetic meridian is inadmissible. I have not, therefore, endeav- 

 ored to interpret the results, but they are given simply as an example of a 

 systematic series of observations, extending over a month. I hope in the 

 summer to resume the work in the absence of the annoyances referred to. 



I may add in conclusion that the experiments referred to above, for measur- 

 ing the gravitational attraction of two identical brass disks, led to curious 

 results. It is easily seen that for constant mass the attraction of nearly con- 

 tiguous disks should increase roughly as the fourth power of their radius. 

 For disks 20 cm. in diameter, however, the result is an invariable repulsion, 

 several times as large as the estimated gravitational attraction, the position 

 of equilibrium being reached gradually in the lapse of several minutes. The 

 subject will be systematically discussed in Chapter II. 



23. Further observations. Film grating. Oil damper. After the above 

 experiment, the steel horizontal pendulum was used for other purposes and 

 observations on the tilting of the pier were discontinued. Later, however, 

 the apparatus was again available and a variety of experiments was made 

 with it. In the first place, the water damper was replaced by an oil damper, 

 as it seemed probable that the surface tension of illuminating oil and its slower 

 evaporation would be an advantage. Under like conditions (though it proved 

 sufficie atly serviceable) it did not check the vibration as effectively as the 

 water damper. The modification of chief interest, however, was the inser- 

 tion of one of Mr. Ives's film gratings (in the usual double plate-glass pro- 

 tection) in place of the plate-glass grating. The film grating in question had 

 about 15,000 lines to the inch, so that the dispersion was excessive, the ellipses 

 being large and diffuse and with a long horizontal axis. To obviate this diffi- 

 culty a thick compensator was introduced into the component beam M passing 

 to and from the micrometer. For this purpose three thick plates of glass 

 were cemented together with Canada balsam to a combined thickness of 

 something over 2 cm. The ellipses now became adequately sharp and almost 

 circular in form. In consequence of the multiple reflections described in Chap- 

 ter IV, Part II, the ellipses are not so strong as in case of the grating ruled 

 on plate glass, and they are much harder to find; but they are nevertheless 

 quite serviceable. The single-plate film grating of 60 was not at hand at the 

 time. It is advisable to try out the double-plate film grating first on the fixed 

 interferometer, in order to determine which lines of the individual images of 

 the slits are to be placed in horizontal and vertical contact, together with the 

 distance which corresponds to the different interferences on the micrometer. 

 After this is done, the corresponding adjustment of the interferometer is easier. 



