191G] on Polarized Light and its Applications to Engineering 588 



presses against the short arm of a lever F pivoted at G, and control- 

 ling the angular position of the lever H of a mirror J pivoted at K. 

 Any change which takes place in the thickness of the specimen between 

 the measuring points causes a rotation of the mirror, and this change 

 can be measured by observing the movement of a spot of light which 

 is reflected from the mirror in the usual manner. The observations 

 can be checked bv the calibrating screw, which is provided with a 

 graduated head Y for this purpose. It will be noticed that the 

 measuring points simply bear against the face of the bar and do not 

 penetrate it, so that the length over which a measurement is made 

 can be accurately determined. The whole of the measuring apparatus 

 is, moreover, supported on a pair of light steel springs M, attached 



Fig. 5. 



to a clip N, secured to the specimen by a pair of screws 0, so that 

 indentations are avoided at the points of measurement, while errors 

 caused by the weight of the instrument on the measuring points are 

 negligible. Further, it may be noted that if measurements of the 

 lateral contraction are compared with those from a similar plate of 

 material when subjected to simple tension or compression stress, it 

 becomes unnecessary to find the values of the physical constants m 

 and E of the material, and a possible source of error is thereby 

 avoided. For investigating cases of plane stress a combination of 

 the two methods described here is chosen, in which the sum of the 

 two principal stresses at a point is found by a lateral extensometer, 

 and the difference by an optical measurement, since it involves 



