382 



Mr. A. Mallock. Some Measures of 



The fulcrum of D is the knife-edge Kj, and a wire passing through 

 A connects the knife-edge K 4 with K 3 . 



The substance to be examined is formed into a small rectangular 

 beam, rather longer than the distance between the fixed knife-edges 

 KI, K 2 , and to the projecting ends of the beam the mirrors M,, M 2 are 

 cemented. These mirrors are mounted in brass frames, and from the 

 back of each frame a small brass tongue, EE' (fig. 2), projects, 

 which is the actual part to which the cement is applied. 



The two other larger mirrors M 3 , M4 are inclined to one another 

 at an angle of 45 nearly. They are fixed in a rigid brass mounting, 

 which rests on the horizontal flat surface FF, from which two studs 

 project, so placed that when the mounting of M 3 , M 4 is in contact 

 with both, the intersection of the planes of MI and M 2 is parallel to 

 the knife-edges K lt K 2 . 



The horizontal width of the mirrors MI, M 2 is less than half that 

 of Mj, MI, and the telescope and collimator are so placed that their 

 axes of collimation graze the vertical edges of M, and M 2 . 



It is necessary that the planes of M t and M 2 should be nearly, but 

 not quite, parallel, and this is effected by cementing the mirrors to 

 the experimental beam whilst the former are held in the gauge shown 

 in fig. 3. 



FIG. 3. 





A spring, not shown in the figure, keeps the mirrors pressed 

 against the plane faces A, A' of the gauge. These are parallel in 

 the vertical direction but inclined to one another about 2' or 3' 

 horizontally, i.e., the planes A, A' intersect at this angle in a line 

 parallel to OY. 



