182 Mr. G. Pierce on Indices of 



viewed through a double glass window, and is further pro- 

 tected by screens from lamps in the room and from the body 

 of the observer. 



The system is suspended by a fine quartz fibre whose 

 force of torsion serves as control. The weight of the system 

 is *27 gram, and the period of swing one way 3 seconds. The 

 resistance of the thermal-junction is about 2 ohms, on account 

 of which large resistance this instrument cannot be made to 

 have the extreme delicacy of Prof. Boys's original radio- 

 micrometer^. It has, on the other hand, a sensitiveness that 

 is not easily obtainable by the use of a thermal-junction in 

 circuit with a galvanometer f. This form of thermal- 

 resonator is not influenced by magnetic or thermal distur- 

 bances, and when once set up requires no further attention. 



The oscillator, for producing the waves, consists, briefly, 

 of two platinum cylinders, tipped with iridium J, and fused 

 for support into the ends of small tubes of thermometer-glass. 

 The oscillating cylinders are each 8 millim. long, 1 millim. in 

 diameter, and are separated from one another by a minute 

 adjustable spark-gap in oil. The mounting of this apparatus 

 is described later. 



The characteristic wave-length of the oscillator and reso- 

 nator is about 4*4 centim. 



There follows an account of the measurement of the indices 

 of refraction, for short electric waves, of certain solid dielec- 

 trics, namely, paraffin, hard-rubber, and various specimens of 

 wood. Some interest attaches to the measurement of the 

 indices of refraction of woods because they prove to be 

 doubly refractive media. 



The measurements are made by observing the distance 

 by which the interposition of a known thickness of the 

 dielectric displaces the nodes of a stationary wave in air. 



Stationary Wave, — The arrangement of apparatus (fig. 4) 

 for obtaining the stationary wave is essentially that of 

 Lloyd's mirror for perpendicular incidence. B, is the radio- 

 micrometer for receiving the waves, the oscillator, and M 

 a plane reflector of commercial copper, 36 centim. square, 

 mounted on a slide on a metre-rod, S, so that it can be dis- 

 placed normally and its position be read off on the metre-rod. 



A part of the electric radiation from the oscillator goes 

 direct to R ; another part is reflected from M to R, and 



* C. V. Boys, Phil. Trans, clxxx. A, p. 159 (1888-89). 



t Tests of sensitiveness, quantitativeness of readings, advantages and 

 disadvantages of this arrangement are discussed in previous paper. 



\ To prevent their burning away, Over 5000 observations have been 

 taken with a single pair of oscillating cylinders thus protected. 



