Convection of Light in Moving Gases. 453 



and it was with the help of these that finally the major part 

 of the aperture was freed from all obstacles. A parallel 

 beam of light now sent down any one of the tubes passed 

 unobstructed and filled almost the whole aperture at the 

 other end. 



The ends of the tubes were closed by windows of about 

 Jin. diameter covered by thick interferometer plates secured 

 between leather washers very much after, the manner 

 employed by Zeeman *', and the current of air was led into 

 the pipe by two side tubes inclined at about 60° to the axis 

 of the pipe. The end pieces for carrying the windows and 

 these inlet tubes were cast in brass, and can be seen in fig. 6 

 (PL VII.) 



This method of leading the current of air into and out of 

 the tubes is calculated to cause the least disturbance in the 

 path of the light. 



The manner of connecting the two tubes with each other 

 at one end and with the arrangement of cocks to facilitate 

 the reversing the direction of flow 7 of air at the other, is 

 shown in figs. 5 and 6 (PL VII.). This connexion is by 

 means of lead pipes, and the reversing arrangement consists 

 of four cocks connected by means of a handle, so that at any 

 time two alternate cocks are open and the other two simul- 

 taneously closed. The central vertical pipe in the photograph 

 leads to the blower, which was used rather as an exhauster, 

 so that the air of the atmosphere entered one of the open 

 cocks of the reversing arrangement, passed through one pipe 

 to the other end, and returned through the other pipe and 

 thence into the blower. This method was followed in order 

 to avoid the temperature changes which would have certainly 

 occurred within the tubes if the compressed air from the 

 blower had been led into them. The temperature of the whole 

 of the air in the tubes was thus kept equal to that of the 

 external atmosphere. 



With all these arrangements complete in January 1921, 

 the two telescopes were carefully adjusted so that a fine slit 

 sent two parallel beams down each tube, and the reflecting 

 mirror at the other end sent them back still strictly parallel. 

 It is evident that the slightest want of adjustment in the 

 direction or the parallelism of the beams was enough to stop 

 all light from passing through the tubes, and even the 

 smallest angle between the two surfaces of each plate closing 

 the ends of the tubes was inadmissible. But when these 

 adjustments were carefully carried out, the fringes obtained 

 were excellent and surprisingly steady, though the light 



* Loc. cit. 



