THE AID OF THE ACHROMATIC FRINGES. 79 



striking the glass under the influence of the 7 rays of radium or in a damp 

 atmosphere can be charged. Yet the phenomenon which shows itself as an 

 accelerated drift toward either plate is very decided, so that the free position 

 of equilibrium to be obtained by gradually adjusting the torsion-head can 

 not be found. 



In view of the importance of this question, I installed a thin quartz fiber 

 of about the same length L = IJ cm. and found its moment of inertia by 

 attaching to the fiber a brass cylinder / = 3 cm. long and m = 2 g. in weight. 

 The period of vibration was found to be 50 to 52 seconds. Hence, as the 

 moment of inertia is 1.5 gXcm. 2 the modulus of torsion is n = o.o24. But 

 even this (by the above method of computation) would not be appreciably 



attracted ,- _. , .. _ B , >. 



(0=1.7X10 6 /o. 024 = 7.1X10 8 rad.) 



by the glass plates; and yet this tendency is marked. 



One may suppose, therefore, that each end of the needle is attracted by the 

 nearer glass plate independently. This will give a superior limit enormously 

 larger than the preceding estimate ; for the force is now 



, if (7 = 3X0.6 = 1.8 



the mean thickness of plate bearing 0.6 cm. Hence the torque is 



since the length of needle is 26 cm., and finally 



= 4.8Xio~ 5 /o.024 = 2Xio~ 3 radian 



Even this excessive estimate, however, fails to account for the result; for 

 the deflection is but a little over a half degree. Moreover, the short brass 

 cylinder in question (length 3 cm.) showed a similar tendency to take oblique 

 positions not corresponding to the torsion-head, as the long needle. 



46. Summer experiments. It is obvious that a fair trial of the apparatus 

 can not be made in an artificially heated room. For this reason experiments 

 in a semi-subterrranean room of the laboratory were reserved for midsummer. 



In the summer installation in a subcellar at constant temperature, with 

 a few improvements of apparatus (the mirrors being readjusted, etc.), the 

 needle was without difficulty made to take a stable position midway between 

 the glass plates, subject to the torsion of the fiber. Some trouble was expe- 

 rienced in finding the fringes, owing to incidental causes. The adjustment is 

 made difficult in view of the definite distance apart of the small mirrors to 

 which the breadth of the ray parallelogram must conform. With the microm- 

 eter at 45 the latter is very limited in its displacement. I later attached 

 a special micrometer with three identical pairs of parallel V-mirrors (the lat- 

 ter at 90) similar to the design shown in figure 2 1 . The middle V-mirror is 

 movable in a micrometer. This, when the mirrors are parallel, has the ad- 

 ditional advantage of being independent of slight changes of inclination 



