142 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



Thus the deflection would be 



4-2X10-* 



s = - = 0.47 cm. 

 9X10- 



which, on being doubled by commutation, is a little short of a centimeter. 

 A tenth millimeter of the scale is thus i per cent. The wire, however, would 

 be capable of sustaining much larger weights and the external mass could 

 be increased at pleasure. 



On the quadratic interferometer with achromatic fringes, if b is the breadth 

 of the ray parallelogram, i the inclination of mirrors to rays, AJV the play of 

 the micrometer, the angle s/2R' of the needle corresponds to 



') = AN cos i 



If b= 10 cm. and 1 = 45, therefore (2(5/2^') =0.94) 



AA T = 



10X0.94 



= 0.023 cin - 



-77Xs8o 



which may be read to io~ 4 cm. directly or to 4Xio" 5 per fringe. With good 

 fringes the accuracy, so far as mere reading is concerned, should be within o.i 

 per cent. 



-/23456T89 



179 



When the telescope and scale are used, the divisions at a long distance are 

 still sharp, but it is often difficult to clearly distinguish the small numbers. 

 For this reason I used the notation shown in figure 179. These dots may easily 

 be put on a glass or other scale in any size, with a sharp pen and india ink. 

 They are very clear at all distances. They have the further advantage that 

 they are equally serviceable, whether the scale be upside down or reversed 

 right to left. 



109. Observations. The chief purpose of the observations in the cold sea- 

 son was to determine the effect of the steam-heated room on the degree or 

 quiescence of so heavy a needle, weighing over 3 kilograms. In the earlier 

 work the absence of iron in the framework was not considered, and the data, 

 though of the same character, were discarded; but the lead-aluminum needle 

 behaved not much better. The hand-drawn steel wire, however, disclosed a 

 second discrepancy of considerable interest, incidentally, as it showed marked 



