552 Mr. H. T. Jessop on Cornu's Method of 



a pair of knife-edges and suspending weights from its ends. 

 He determined the longitudinal and transverse curvatures by 

 measurement of the fringes along and across the beam. The 

 value of Young's Modulus for the glass is readily found 

 from the longitudinal curvature, while the ratio of the 

 transverse to the longitudinal curvature gives the value of 

 Poisson's Eatio for the material. Cornu also mentioned 

 that Poisson's Ratio is given directly by the square of the 

 tangent of half the angle between the asymptotes of the 

 hyperbolas. 



Cornu's determinations are open to one grave fault which 

 seems hitherto to have escaped comment. In four of the 

 six determinations quoted in his paper, the widths of the 

 beams employed were 254, 2*50, POO, and 1*30 cui. ; while 

 in each case the distance apart of the knife-edges was only 

 1*60 cm. That is to say, that in the most favourable case 

 the width of the beam was greater than the distance from 

 the centre of the beam to either knife-edge. Thus no part 

 of the beam was sufficiently far from the point of application 

 of the forces to satisfy the conditions that de St. Ven ant's 

 principle of Equipollent Loads could be applied, and the 

 changes in distribution of pressure along the knife-edges as 

 the beam was deformed must have had considerable effect on 

 the bending in the region observed. The other two results 

 quoted are free from this fault, for the distance apart of the 

 knife-edges is given as 12 cm., while the widths of the 

 beams are 1*35 cm. and 3'30 cm. 



The ratio of width to thickness of the beams he employed 

 ranged from 3*76 to 18'40. 



(2) A modification of Cornu's method was employed by 

 Straubel in 1899 *, for the determination of Poisson's Eatio 

 • for a number of optical glasses. Straubel applied the 

 bending-moment to the beam by means of two pairs of knife- 

 edges, the force being applied by a screw and being left 

 unmeasured. He determined Poisson's Eatio by measure- 

 ment of the angle between the asymptotes. 



In an extensive series of tests on one particular glass, the 

 specimens nsed ranged from 1 cm. to 3 cm. in width, while 

 the distance between the inner knife-edges varied from 

 3*4 cm. to 7 cm. The ratio of width to thickness of the 

 beams varied from 5 to 10. The following table is a summary 

 of the mean values obtained for this series of tests. 



* "Ueberdie Elasticitatszahlen mid Elasticitatsn. oduln des Glases," 

 Arm. der Phys. u. Chem. lxviii. p. 369 (1899). 



