452 Prof. 0. V. Raman and Dr. N. K. Sethi on th 



on tne 



Morley's experiment the size o£ the apparatus was very 

 much smaller and the velocity o£ air not more than about 

 25 metres per sec, so that the observed shift was, as stated 

 by them, certainly less than T ^ of a fringe and probably 

 less than ^Jo- 



The actual setting up of the apparatus was taken in hand 

 about July 1920, and the site chosen was the compound of 

 the Indian Association. One of the out-houses served to 

 house the optical parts at the observing end, and a small hut 

 was erected at the other end to receive the reflecting arrange- 

 ment L 2 R (fig. 1). The big telescope body with the object- 

 glass was mounted on solid brick-work pillars erected for 

 the purpose, and adjustments had to be provided for both 

 vertical and horizontal movements. Some of these are 

 shown in fig. 3 (PL VI.). A fine spectrometer slit was 

 mounted at the eyepiece end of the telescope, and the thin 

 plate-glass, P, was also mounted in the same tube. A micro- 

 scope was used to observe the fringes, For the other end, 

 another shorter telescope (4 inch objective) was similarly 

 mounted on two pillars, and the eyepiece was in this case 

 replaced by one of the front-silvered Michelson inter- 

 ferometer mirrors, so mounted as to allow of accurate 

 adjustment of its plane. But the real difficulty lay in the 

 fixing of the tubes. Galvanized-iron pipes of 1^ in. internal 

 bore were selected, and each piece was carefully straightened. 

 Brick pillars were erected at intervals of about 10 ft., and 

 their tops carefully aligned by means of a theodolite so that 

 the deviations of the top line from the mean did not exceed 

 T l of an inch for any individual pillar. This having been 

 accomplished, the pipes were assembled and placed on top 

 of the pillars, so that ultimately there was a pair of tubes 

 quite straight from . end to end and separated from each 

 other by a constant distance throughout. There was no 

 appreciable sag of the pipe between the pillars. This seems 

 to have been avoided automatically by having such a con- 

 siderable continuous length. This was finally tested by 

 placing a small hole near one end and illuminating it by a 

 strong beam of light. Observing at the other end, the 

 whole of the aperture should be uniformly illuminated if 

 there was no bend anywhere. Unfortunately this was very 

 difficult to secure, for the slightest cntting-off of the light 

 anywhere cast a tremendously large shadow at the great 

 distance where it was being observed, and it was not at all 

 easy to locate where the fault lay. The width of the diffrac- 

 tion fringes at the edge of the shadow, however, gave a rough 

 and ready indication of the approximate position of the fault, 





