somewhat analogous to Newton s Rings. 75 



Experiment III. Search for Interferences as in Experiment I, lut with 

 the Abnormal Part away from the middle of the Secondary. 



As the outcome of Experiment II the theory resulting in condition 

 (B), p. 71, was developed, and the present experiment tried in 

 accordance therewith. The effective length of the wave- train was 

 experimentally found to be about 70 m. Hence, to fulfil (B) it was 

 necessary to make (SAM * MD) >35 m. 



The apparatus shown in fig. 1 was then arranged thus: SAD = 

 164 m. SAM varied from 95*25 m. to 105*75 m., and electrometer 

 throws were taken alternately with and without a pair of tinfoil sheets 

 as in Experiments I and II. 



This experiment was performed twice and the mean result, when 

 plotted as in Experiment 1, gave a curve undulating between the 

 limiting ordinates 0'55 and 0'46. Thus the extreme values of the 

 ordinates differed from their mean value .by about 9 per cent, of the 

 latter. 



It is thus seen that, although the straight line hoped for was not 

 obtained, yet this curve is less wavy than either of the two previous 

 ones. And since, also, this arrangement yields the maximum effect at 

 the electrometer, it was adopted in the experiment for the analogy to 

 Newton's rings by transmission hereafter described. 



Experiment IV. Estimate of the Hate of Decay of the Waves in their 

 Advance along the Secondary. 



The arrangement of the apparatus adopted for this determination 

 has already been described under the headings of Theory (p. 72). 

 In this experiment the droop of the curve sought was expected to be 

 very slight. Consequently, for the sake of accuracy, each point of 

 the curve was determined by twenty-one electrometer readings. Six 

 sets of observations were made with SAE, fig. 1, = 160 m. They 

 yielded a curve whose asymptote was 



y x = 0'34 ; whence we obtain 



<r = 0-0018 (7). 



A second series of observations with SAE = 91'5 m. afforded the 

 values y x = 0'31 and 



a= 0-0029 (8). 



Experiment V. Analogy to Newton's Rings by Transmission under lest 



Conditions. 



This experiment was made with exceptional care, as it is the chief 

 one of the series. Most of the others either served to indicate the 



