402 Prof. W. Cassie on the 



being applied to the end of the spectrometer tube, while 

 the slit was illuminated with a Nernst lamp, which makes 

 the best source of light possible when a continuous spectrum 

 is required. The photographs are interesting as showing 

 the sharpness of the bright and dark bands in the spectrum- 

 I am of the opinion that a study of the colours of the central 

 image with polarized light in the case of this grating may 

 throw some further light on the problem, which is one of the 

 most interesting that I have ever met with. 

 Baltimore, June 2nd, 1902. 



XLIII. On the Measurement of Young's Modulus. By W. 

 Cassie, M.A., Professor of Physics in the Royal Holloivay 

 College *. 



A reliable oscillation method of measuring the stretch 

 modulus ought to have advantages in accuracy and 

 convenience which would give it some practical value. A 

 method depending upon the oscillations of a spiral spring 

 has been given by Prof. L. R. Wilberforce *, and a simplifi- 

 cation of that method depending upon flexural vibrations of a 

 straight piece of wire has been given by Mr. G. F. C. Searle f» 

 The apparatus described in the present paper yields an 

 oscillation method which is fairly simple, and it has the 

 additional advantage that without any change of the apparatus 

 a statical measurement of the stretch modulus can also be 

 easily made. 



If a horizontal bar AB, fig. 1 (hereafter called the needle), 

 is symmetrically supported by two equal parallel wires pq, 

 rs it may be made to execute a small oscillation in the plane 

 of the paper about an axis passing through the middle point 

 of qs perpendicular to the plane of the paper. This oscil- 

 lation is accompanied by alternate extension and contraction 

 of the supporting wires, so that the resistance to stretching 

 of these wires controls the oscillation and determines its 

 period. The period of the oscillation for a given pair of 

 wires may be made of any convenient length by altering the 

 moment of inertia of the needle. Some of the dimensions of 

 the apparatus can be eliminated by observing other modes 

 of oscillation of the system, so that in its simplest form the 

 experiment gives an expression for the stretch modulus 

 involving only four periods and the weight of the needle, 

 qnantities which can be measured with ease and accuracy. 



* Communicated by the Physical Society : read November 22, 1901. 

 t Phil. Mag. Oct, 1894.- % Ibid. Feb. 1900. 



