76 DISPLACEMENT INTERFEROMETRY BY 



rapidly than the nearer ray from M' and therefore parallelism with local 

 coincidence of rays on M' is generally possible. If the parallel rays T\ 

 and T 2 which coincide in T are too far apart, no fringes can be found, even 

 when the path-difference is annulled. 



44. Heavy needle in air. The needle was now deprived of its floats and 

 other appurtenances, provided with somewhat lighter balls (w=i8 grams 

 each), and suspended in the same float-vessel in air. Since in this case AAf 

 is independent of m, and as the dimensions were about of the same order 

 given in the example above (//' = o.c>5 cm., L=5o cm., M=io 3 g., 2R=io 

 cm., /" = 3o cm.) with a somewhat larger d = 6 cm., AN = 0.0004 cm. per 

 kilogram of attracting load was to be expected; i. e., about 10 achromatic 

 fringes. 



The adjustment is quite difficult, since the small mirrors must not only 

 be approximately parallel, but must be so spaced and inclined as to receive 

 the component beams and to reflect them through the mirrors of the inter- 

 ferometer. This was accomplished with some patience and the achromatic 

 fringes were found. But here again they proved to be in incessant and rel- 

 atively rapid motion, so that they swept continually through the field. 

 Though observed for some time, on different days, they were never found 

 .sufficiently quiet to admit of the above measurements. It was impossible, 

 in other words, to eliminate the air-currents within the shallow envelope 

 sufficiently to warrant counting at the rate of 10 fringes per attracting 

 kilogram. In the summer I think this would have been feasible, as there 

 was no other drawback militating against the completion of the experiment. 



45. Light needle in air. In view of the failure of the heavy needle, I went 

 to the opposite case of a relatively light needle, weighing when loaded 

 but 1.49 grams. This was made of a rigid shaft of straw (mm', fig. 54) 

 25 cm. long, the ends being slit slightly into four symmetrical segments 

 each, which receive the two shots m and m', additionally secured with a 

 little wax. The two light mirrors b and c were differently mounted; b on 

 a fine pin d, snugly fitting corresponding perforations in the straw, was 

 thus capable of rotation around the vertical axis (d) and moving up and down 

 slightly. The mirror c, however, was mounted on a thin elastic strip of alu- 

 minum, clasping the shaft as shown in section at e'c', and thus capable 

 not only of rotating on a horizontal axis, but of being placed at different 

 distances (moving right and left) from d to accommodate the rays of the inter- 

 ferometer, to which b and c are to be normal. The needle is swung from a 

 quartz fiber q and a strip or hanger of elastic aluminum a (see a',q'), which 

 clasps the shaft. Hence the latter may also be moved endwise to be balanced 

 and rotated around a horizontal axis till b and c meet the rays normally. 

 These operations are completed by trial before the needle is definitely hung, 

 preferably in a broad beam of sunlight. No great accuracy is required. 



The shallow case is made of two plates A, A' (figs. 55, 56) of Ys-inch plate 



