684 The FitzGerald-Lorentz Effect. 



four mirrors each are fastened by bolts through their sup- 

 porting frames. Against these holders rest eight slender 

 pine rods, supported throughout their lengths by enclosing 

 them within the tubes of a brass truss. Against the further 

 ends of these rods are held the freely suspended holders of the 

 other two sets of mirrors. Springs apply a certain force to 

 hold the freely suspended holders against the rods, and so 

 against the first fixed mirror-holders, so that the distance 

 between the mirrors depends entirely on the pine rods. In 

 fig. 3 (PL X.) the apparatus is shown in an incomplete state; 

 the telescope is shown in its final position, but the lamp 

 and condensing-lens are not in place. Fig. 4 (PI. X.) 

 shows the apparatus ready for observations ; with the lamp 

 and lens placed as far as may be from the optical parts of the 

 apparatus, and shut off somewhat by screens. The whole path 

 of the light in the apparatus is enclosed, and the observer is 

 protected by a dark cloth from the light w T hich is necessary 

 for the recorder. 



With this apparatus, we adjusted our fringes on a certain 

 Monday, and found that they remained in adjustment 

 throughout the week during which we were occupied in 

 observing them. 



With this apparatus, observations were made precisely as 

 before. We obtained 260 complete observations consisting 

 each of readings at sixteen azimuths around a circumference. 

 At the date of the observations, the annual motion of the earth 

 together with the motion of the solar system may be taken as 

 33*5 kilometres a second. The velocity of light being 300,000 

 kilometres a second, the ratio of the squares of the velocities 

 is 0*72 . 10 8 . The length of path of a ray in our apparatus 

 was 3224 centimetres, in which distance there are contained 

 5*5 . 10 7 wave-lengths of sodium light. The expected effect 

 being doubled by rotation through 90°, the displacement of 

 fringes expected on the simple kinematic theory will be 

 11 . 10 7 -r0-72 . 10 8 . This is 1-5 wave-length. 



As was indicated, there were two times in the day when 

 observation was advisable. The direction of the motion with 

 reference to a fixed line on the floor of the room being com- 

 puted for the two hours, we were able to superimpose those 

 observations which coincided with the line of drift for 

 the two hours of observation. Doing this, and subtracting 

 a constant so as to make the algebraic sum of the observations 

 equal to zero, we get a certain result. Then adding the 

 first term to the ninth, and so on, since the effect repeats 

 itself after a half revolution, we get our final result, as 

 follows : — 



