THE AID- OF THE ACHROMATIC FRINGES. 



65 



anticipated, a priori, although the amount of this quality is also modified 

 by the yield of the apparatus. If hysteresis is marked, the moduli E would 

 naturally increase with increasing loads as the top ends of the loops become 

 relatively more horizontal. One would expect, however, that the curve with 

 continually increasing loads should be straight, and the curvature (concave 

 downward) of the above graphs is probably introduced by the apparatus. 



171 



46 



d 



&' 



Jf 



fef 11 * 



'4 



I therefore modified the apparatus as shown in figure 45 (plan) by attach- 

 ing the offsets 55 and s's' firmly to the sheaths b and b' (holding the rods r, 

 /), by means of set screws. The pull of the pulley strings p and p' is to be 

 coaxial with the respective rods r, r'. FF' is the bar holding the auxiliary 

 coplanar mirrors m, m' and is supported by the bifilar threads attached at 

 d and d' . This bifilar is now also to be advantageously modified (fig. 46, 

 side elevation) by doubling it and attaching 

 the tense fibers d,d', above and below at 

 and e' to corresponding standards. There are 

 such pairs of fibers at either end of FF'. Thus 

 the bar is held in place in the absence of the 

 stress along p f , and this should be applied in 

 such a direction as not to disturb the reflec- 

 tion from the mirrors m',m. 



36. Observations. Cycles obtained with the 

 new apparatus are given in figures 47 and 48. 

 They are in many respects satisfactory, show- 

 ing the hysteresis loops and the gradual yield- 

 ing of the viscous solid to stress. In spite of 

 all precautions, however, the slit-images sepa- 

 rated very slightly, proving that the rigid 

 attachment of the offset 55 (figs. 45, 46) pro- 

 motes flexure as the result of small cross- 

 torques. The loose offset which can not convey flexural torque is in this 

 respect preferable. Of the values of the modulus computed from the 

 pressure ascending branch of the cycles the following data may be re- 

 corded by way of example (hard-rubber rod, 2L = 5.o5 cm.- 2r = o_37 cm.; 

 A=o.ioy cm. 2 ). 

 5 



