Convection of Light in Moving Gases, 451 



path necessary, the attempt to observe the fringes with it did 

 not prove to be a success. A large refracting telescope with 

 a 7 inch object-glass and a focal length of over 7 feet was 

 available from the observatory of the Indian Association for 

 the Cultivation of Science, and was made use of in the work. 

 The plan of the optical arrangement is shown in fig. 1, from 



Fig, l. 







:v-h 



w hich it is clear that if I be the length of each of the tubes 

 B and CD, the shift of the fringes observed when a 

 c urrent of air is sent round with a velocity v in the direction 

 °f the arrows will be 



Uv0 

 XV 



of the distance between two consecutive maxima or minima, 

 where 6 is the convection coefficient and V is the velocity of 

 light in the medium. If, however, the current is first 

 allowed to flow as in the figure and then suddenly reversed, 



the total shift will be double this or equal to — ^ . It is 



A V 



evident, therefore, that what is required in order to increase 



the shift and render it measurable is that both I and v 



should be increased. Consequently we proposed to utilize 



all the space available to us and make the tubes about 



200 feet in length. For driving the current it was proposed 



to utilize a Pressure Blower driven by a 3-h.p. gas engine 



{fig. 3, PL VI.). With this we expected a velocity of 



air of over 50 metres per sec. in the tubes. The amount 



of shift of the fringes which we could therefore expect 



became 



8x200x30x5000x (1 



(1-0003)V 



5xl0-°x3xl0 1 



or about one-tenth of a fringe-width, w 



.measurable with 



sufficient accuracy. 

 2 G2 



hich should be easily 

 In Michelson and 



