420 Mr. J. Morrow on an Instrument for 



by the bars E, F, and the pivot P, to the extremities H, G, 

 of the pieces K, L. 



PLAN 



V.3 



ELEVATION 



ScAUE Half ^i^e 



It will be noticed that the instrument touches the specimen 

 only at the points between which the alteration o£ dimension 

 is to be measured. When the specimen extends A and B 

 approach one another, and consequently G falls with re- 

 ference to H. The relative displacement of G and H is 

 measured optically. A mirror M rests by three steel points 

 (two of which are on G and the other on H, as shown in the 

 plan of the instrument) on glass plates fixed to G and H. 

 A second mirror N is attached to G in a vertical position, it 

 is capable of a small amount of rotation about a vertical axis 

 by the fine adjustment screw Q. The two mirrors are close 

 together but not touching. A scale is placed some distance 

 away, and by means of a telescope two images of the scale 

 are seen, one in each mirror. These images are brought 

 together in the telescope and the one is read against the 

 other. Thus a certain convenient reading on the JS^ image is 

 taken as the index, and as the load is applied to the specimen 

 the observer reads the values on the M image coinciding with 

 luis index. 



Every precaution has been taken to ensure the greatest 



