ACOUSTICS AND GRAVITATION. 



75 



be the index in measuring the fringe displacement. If the telescope with 

 cross-hairs is so to be used, this must also be rigidly attached to it. I used the 

 former method in preference. 



After the stops t and t' (fig. 102) to the index a have been fixed, the measure- 

 ment consists in moving the micrometer P at N', until the fringes coincide with 

 the guide-wire across the slit -image. This wire should be very fine. If the 

 micrometer-screws at t or t' are moved, the images of the cross-wire and fringes 

 both move, preferably in opposite directions ; but the fringes move several- 

 hundred times faster, so that the slit-image is relatively stationary, or can just 

 be seen to move. 



In the first experiments a phosphor-bronze wire 0.025 cm - i n diameter and 

 about 35 cm. long was inserted. This was adapted for small weights of the 

 order of a few centigrams. The double deflections here amounted to about 

 1.7 per milligram and measured about 5 mm. on the scale 5. As a rule, how- 

 ever, thicker wire and larger weights will be preferable, contributing to a more 

 robust apparatus. 



As to dimensions of the apparatus constructed, I may add that the beam 

 cmm'c' was 22 cm. long, the disks v about 5 cm. in diameter, and the distance 

 cv about the same. The breadth b of the ray parallelogram (fig. 100) was 10 

 cm. The apparatus stood about 50 cm. high and should have been covered 

 with a case to obviate air-currents, in addition to the case CC. 



The wire is to be kept free from twist, except when used. The degree to 

 which the twist, and therefore the sensitiveness, may be increased without 

 an error from viscosity must be left to trial. 



In the earlier apparatus the balance was rigidly attached to the base A. 

 In the later forms with thicker wires of steel, the balance and appurtenances 

 were mounted on an independent framework of iron, which could then be slid 

 underneath the interferometer, as shown in figure 100. This makes it much 

 easier to insert new wires w, and adjust the mirrors mm' to approximate paral- 

 lelism, by aid of a beam of sunlight or the like. 



In the above apparatus caps on pivots were chosen at the scale-pans, merely 

 for convenience of construction. It is obvious that knife-edges and plates 

 would, in general be preferable, by reason of greater stability. 



With the air damping above the apparatus is practically aperiodic for thin 

 wires (0.02 cm.) and the beam takes its place at once. It is still sufficiently 



