298 Mr. E. W. B. Gill on the Electrical 



because it would have to be obtained by combining Rubens* 

 curve (fig. 1) with fig. 4, and his measurements do not go as 

 far as the electrical ones ; it would be in addition slightly 

 misleading because for any particular value of n we have a 

 mixture of two wave-lengths corresponding respectively to the 

 ordinary and extraordinary rays, though for quartz these wave- 

 lengths are nearly equal and the extraordinary ray is of very 

 feeble intensity. 



The curve given represents the mean of a number of 

 observations, but observations made at different times showed 

 variations greater than experimental errors which have not 

 yet been explained. There are three possible causes of 

 variation, the state of the zinc plate may vary, the light 

 emitted by the spark may vary, or the absorption by the air 

 may vary. Taking these in order it is true in the first place 

 that a freshly polished zinc plate is very active in giving off 

 ions, and as it gets dirty, i. e. oxidized, the number of ions 

 given off (other things being equal) decreases, but a steady 

 state is soon reached. The only thing which appears to 

 disturb this steady state is the passage of a spark-discharge 

 from the wire gauze to the zinc plate, but even then the 

 original steady state is regained after 3 or 4 hours. In the 

 second place the light from the spark-gap was kept as constant 

 as possible by always having the same current through the 

 primary, the same length of spark, and approximately the 

 same adjustment of the hammer interrupter. On the whole 

 it is improbable that either o£ the above causes is responsible 

 for the variations, a supposition that is partly confirmed by 

 the fact that on any one day the readings were consistent, the 

 variation being from day to day ; and it seems likely that 

 the absorption by the air is the determining factor. The 

 published results for air-absorption by different investigators 

 are very inconsistent, and it may be that the absorption is 

 more due to the moisture in the air than to the air itself, 

 which would explain the variation from day to day. 



A study of the curve obtained (fig. 4) shows that the 

 electrical effect is just beginning about the middle of the 

 visible spectrum and increases up to a first maximum for 

 refractive index n= 1*66, sinks to a minimum at 1*67, and 

 reaches a second maximum for w — 1*68, rapidly decreasing 

 after that. The position of the minimum was always exactly 

 the same for every experiment and the second maximum at 

 71 = 1*67 was fairly steady, but the first maximum on different 

 occasions varied, sometimes appearing at 71 = 1*65. In this 

 case besides the above considerations there is an additional 

 explanation of this. Rubens' curve shows that in the 



