1894.] 



Experimental Determination of Poissons Ratio. 

 FIG. 2. Instrument B. 



375 



m *; 



X 



the frames must account for this large difference, namely, 24 per 

 cent., over the true value of O'OOOOllGin. Although the cause might 

 be known, this large correction introduced an element of uncertainty, 

 which the author hoped to eliminate by constructing a new instru- 

 ment, B, fig. 2. 



In this sketch, T is the section of the test piece, which is pressed 

 against the point on the frame FH by the screw S. G is the dark 

 glass, which, as soon as T contracts, is pulled away from the glass 

 prism P by means of the four helical springs Z, Z, which surround 

 the columns C, C, and which are firmly secured to the frames F a , F 3 . 

 The latter carry the adjustable glass prism P, which is so shaped that 

 the ray of yellow sodium light L t does not fall together with its 

 reflected ray L 2 . The inclination of the rays of light in the narrow 

 space between the prism P and the dark glass G was carefully 

 measured, and found to be 19, so that each interference band, as 

 seen in the reflected yellow light, ought to represent a distance of 

 O g 00001 09 in., but careful measurements with the fine screw S showed 



