Interference Bands formed by Rays at Small Angles. 189 



The tape is passed over the rockers Ri, R2, as in fig. 2,* and is kept in 

 constant tension by the spiral springs Si, Sg, one of which is hooked on to a 

 ring at the end of the tape, and the other to an adjustable clamp, gripping 

 the tape at an appropriate place. To each rocker an arm is attached, 

 carrying a cylindrical stud, Hi, H2. Thus any expansion or contraction in 

 the girth of the tree causes the distance between the studs to increase or 

 diminish, the friction between the tape and rockers under the tension of the 

 springs being quite sufficient to prevent slipping. 



The optical part of the apparatus (figs. 2, and 3) hangs freely from an 



Fig. 2. Fig. 3. 



arm A, projecting from the stout plate on which the rockers are mounted. 

 The plate itself is attached to the tree trunk by screws. P is a right-angled 

 glass prism mounted on a support C, capable of turning about the axis D in 

 the outer frame B, and the angular position of the prism, with reference 

 to B, can be adjusted by the micrometer screw ,G. Q is a flat glass plate, 

 blacked on the hind surface, and having a straight edge at F. The plate is 



* In fig. 2 the tape is replaced by a thread, so that the rockers may not be hidden 

 from view. 



VOL. XC. — B. O 



