374 



Mr. C. E. Stromeyer. 



[Apr. 1! 



bo nearly parallel, and, on throwing yellow sodium light into th 

 prism, interference bands could be seen in the reflected light, an 

 these would move either in one direction or the other, according a 

 to whether the two glass surfaces, and with them their two frame 

 were either moving towards or away from each other. By conn tin 

 the number of interference bands, which passed a mark which 

 been scratched on the dark glass, it was possible to estimate 

 amount of the relative motion of the two glass surfaces, each ba 

 representing a motion of half a wave-length of sodium light, or abo' 

 O0000116 in. A centre point projected from the under-side 

 each frame, and these could be pressed against that part of 

 structure where it was intended to measure the variations of st 

 Subsequently these centre points were replaced by two 

 brackets and set screws, and in this form the instrument has 



FIG. 1. Instrument A. 



used in the following experiments. Fig. 1 shows a section thi 

 the instrument as altered, Fj and F 2 are the two frames, S h S i 

 the flat springs holding them together and keeping them paralle 

 G is the black glass, P is the right-angled reflecting prism, and L 

 ray of sodium light. B! and B* are the two brackets, and T is 

 section of the test piece in position and ready for testing. 



It was soon found that the results which were obtained with tl 

 instrument differed materially from those which were obtained 

 less direct methods ; it was therefore taken to South Kensington 

 calibrated in a Whitworth measuring machine in company 

 Mv. Boys, by carefully comparing the relative motion of the 

 screws a and b, fig. 1, with the number of interference bands whi( 

 had passed the mark on the dark glass. It was found that each 

 represented 0*0000144 in. Evidently the spring of the brackets 



