52 Journal of the Asiatic Society of Bengal. [February, 1914. 



Description of the Apparatus. 



The rod AB (Fig. I) which is of greater diameter between 

 E and F is capable of turning with as little friction as possible 

 between two screw bearings S, S'. In the middle of the por- 

 tion EF is drilled centrally a hole through which passes a 

 glass rod which can be clamped by means of the screw P. 

 Between E and A is a collar which can be clamped in any posi- 

 tion, and carries a concave mirror which throws an image of a 

 spot of light on a distant vertical scale. 



The left-hand portion of the apparatus (Fig. I) is the part 

 which is clamped to the experimental wire. It consists of two 

 brass plates C and D which can be fixed firmly to the wire 

 by means of four screws. The plate D carries at right angles 

 (as shown in the figure) a plate L. To the centre of the 

 plate C and at right angles to it is soldered a rod carrying a 

 sliding weight G which can be adjusted in any desired position 

 on the rod and serves as a counterpoise. On the inner surface 

 of the plate D is cut a very fine vertical groove in such a posi- 

 tion that when C is clampe 1 and G properly adjusted, the 

 centre of gravity of the whole system lies in this groove. The 

 groove is such that the wire passing along it is rigidly clamped 

 when the four screws are made tight, but on loosening the 

 screws the plates can be made to slide along the wire with just 

 sufficient friction to support them, the groove guiding the 



plates vertically. 



The glass rod is either clamped at a point some distance 



from its centre or is slightly weighted at one end, so that 



its centre of gravity lies to the right-hand side of P. Thus 



it always presses against a steel knife edge fixed at the edge of 



a slit t. 



W 



plates descend, and the projecting piece descending vertically 



* * * t 1 * * * * *-ih* ^n_* mm mi Am 



rot at m 



the mirror M. The spot of light which is received on a verti- 

 cal scale is thus displaced upwards. (The magnification that 

 can be obtained is as much as 200). 



Uniformity of Magnification. 



Preliminary experiments were made, to test whether the 

 magnification was uniform over the scale. The clamped plates, 

 detached from the wire, were made to descend vertically by a 

 micrometer screw fixed relatively to the ground, thus turning 

 the rod as in the previous case ; the amount of descent being 

 read 08 from the graduations of the screw and the deflection 

 of the spot of light on the scale being noted, the magnification 

 of the instrument could be determined. It was found by this 

 method that when the scale is about two metres from the 



