186 DE. EVEEETT ON THE EIGIDITY OF GLASS. 



G K, H L are light arms of brass clamped to the glass rod by screws G, H, at the 

 distance of several diameters of the rod from the sockets A, B, and carrying at their 

 other extremities the semicircular mirrors M, N, which are represented on a larger scale 

 in figs. 4, 5. Each mirror is attached to its supporting arm by a vertical rod a i, turning 

 in a socket at a, and having a joint at h, by means of which the plane of the mirror can 

 be turned to or from the vertical. By these two motions we can cause the image formed 

 upon the screen to move either horizontally or vertically. The mirrors always face the 

 screen. In flexure experiments their planes are perpendicular, and in torsion experiments 

 parallel to the length of the glass rod, as shown in figs. 4, 5 respectively. P P, Q Q are 

 rectangular frames of iron for supporting the apparatus. 



Fig. 1 exhibits the general arrangement for flexure experiments. A is the apparatus 

 above described, D a paraffin lamp, C a frame for supporting a fine brass wire stretched 

 horizontally on a level with the middle of the flame, and nearly in contact with the glass 

 shade of the lamp. As seen from the mirrors, this wire appears as a fine dark horizontal 

 line passing through the flame. B is an achromatic lens of 4 feet focal length, mounted 

 on what is equivalent to a universal joint. The rays of light from the flame and wire 

 pass through this lens to the mirrors, and after reflexion pass outside the lens to the 

 screen E, on which two images of the wire (one from each mirror) are formed, appearing 

 as horizontal lines of darkness in the midst of large spots of light. The clearest portions 

 of the two images were brought, by adjustment of the mirrors, into the same vertical 

 line, and the distance of their centres was directly measured with a rule divided at the 

 edge to millimetres. The images were sufficiently narrow to be easily bisected by eye, 

 and the readings were taken to tenths of a millimetre. 



In torsion experiments, the apparatus A was placed so that the glass rod was parallel 

 instead of perpendicular to the screen, but the mirrors were turned so as still to face the 

 screen, and the appearance of the images was the same as above described. In both cases 

 the images could be made either to approach or recede from one another on hanging on 

 the weights, by adjusting the mirrors so as to cause one or the other of the images to be 

 in the first instance uppermost. In some of the experiments they approached, in others 

 they receded. In all the experiments they were nearly on a level with the mirrors, and 

 the rays, both direct and reflected, were nearly perpendicular to the screen. 



The mirror-arms G K, H L, fig. 2, were so adjusted that one of the mirrors was a little 

 behind, and at the same time a little to one side of the other, as represented in figs. 4, 5 ; 

 the former being a side and the latter a front view. They were just far enough apart in 

 both directions to prevent any risk of their coming in contact when flexure and torsion 

 were produced in the glass rod. 



As the portion of the rod whose flexure and torsion are measured is that which lies 

 between the clamps G, H, it was necessary to ensure that these should always be in the 

 same places ; and to this end two measuring sticks, cut to convenient lengths, were em- 

 ployed, and whenever it was necessary to unscrew and readjust the clamps, the distances 

 between B and H and between H and G were made to fit these sticks. 



