THE AID OF THE ACHROMATIC FRINGES. 83 



II. USE OF THE RECTANGULAR INTERFEROMETER IN CONNECTION WITH 



THE HORIZONTAL PENDULUM. 



47. Introductory. In 1915 and in the reports of the Carnegie Institution of 

 Washington, No. 229, Chapter I, part 2, pp. 30 et seq., I adduced a method for 

 the application of the displacement interferometer to the horizontal pendulum 

 with a graphic exhibit of the results obtained during a series of months. 

 The concave-mirror design by which the spectrum interference ellipses were 

 made available showed a very satisfactory performance, in spite of the fact 

 that deformations of the pier to which the pendulum was attached were local 

 disturbances and excessive in amount. The attainable accuracy was such 

 that for moderate constants in the installation of the pendulum, an inclination 

 of 3Xio- 4 second of arc should have been registered per vanishing interfer- 

 ence fringe (ellipse), or about icr 3 second per io~ 4 cm. of displacement of the 

 micrometer. The inclination of the line of suspending pivots was here about 

 i to the vertical. A smaller angle would have correspondingly increased the 

 sensitiveness. 



The apparatus, however, required a space about 2 meters long between the 

 extreme mirrors for its installation. This is in a measure a disadvantage, 

 since small changes of temperature in the brackets and supports, as well as 

 in the pier, would interfere with the full realization of the precision of the 

 method. In this respect the rectangular interferometer with an auxiliary 

 mirror is to be preferred ; for here all the necessary parts may easily be placed 

 within a distance of i foot from the wall of the pier carrying the horizontal 

 pendulum. If the achromatic fringes are used, these are straight and in- 

 tense, so that photographic methods are available, while for visual observation 

 a gas flame would give sufficient light. The sensitiveness under similar con- 

 ditions would be slightly smaller, but not enough to cancel the advantages 

 specified. 



48. Apparatus. The old horizontal pendulum formerly described was 

 again used. It was made of thin steel tubing, and in this respect, since its 

 plane was nearly in the meridian, may be subject to change of the earth's 

 magnetic field; but as my object here is merely the trial of a method, these 

 annoyances are of slight consequence. 



Figure 57 gives a sectional plan of the pendulum installation and figure 

 58 a front view of the pendulum HH alone, on a somewhat reduced scale. 

 Its general shape is that of an isosceles triangle and the distance from the 

 line of pivots tt' to apex B about no cm., while the distance between pivots 

 was 97 cm. The pivot supports 55' are fine screws ending in hard-steel 

 points, which enter a glass-hard steel socket (below) and a steel groove 



