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



entering the slit be the portion between n -\-^dn to n— \dn it 

 is easy for the particular dimensions of the apparatus used 

 to show that 



dii 



= -~q£~\/ 1 — "a approximately, 



the lenses being properly focussed and the prism having an 

 angle of 60°. The extreme observations were given by 

 /i=l*56 and w=l'7l, and the corresponding values of dn 

 were *009 and *0105 ; it is thus seen that da never differs 

 much from '01 and as readings were taken for n = 1*56, 

 1*57 .... 1*71 the whole spectrum was embraced in these 

 16 readings. A few intermediate readings were also taken 

 near points of interest on the resulting curve. 



It must be borne in mind that although the range of 

 spectrum dn gathered in is approximately constant yet the 

 number of different wave-lengths d\ gathered in by the slit 

 is by no means constant. 



The apparatus being properly adjusted for minimum 

 deviation for refractive index n the light was turned on for 

 10 seconds and the corresponding deflexion of the electro- 

 meter noted. The prism was then turned through some 

 angle a to the position corresponding to some other index of 

 refraction n' and the detector was turned through 2a, thus 

 being put in the minimum deviation position for n\ the 

 lenses were refocussed for refractive index n' and the deflexion 

 again noted in 10 seconds. The ratio of the two readings 

 gave the relative number of ions given off by the two ranges 

 of spectra n— *05 to n + '05, and n' — *05 to ?i' + *05. Care 

 must be taken lest in the second focussing we alter the cone 

 of light received by the apparatus from the spark. To keep 

 this constant S, B, and C (fig. 2) were kept fixed in space ; 

 and in order that the length of air traversed by the rays 

 should be constant G was kept at a fixed distance from the 

 prism. 



To prevent cumulative errors three standard positions were 

 taken corresponding to values of n of 1*57, 1*61, and 1*65 

 respectively, and the ratios of the numbers of ions set free 

 at these very carefully determined. All other ratios were 

 referred to one or other of these positions ; for example, the 

 effect at ft = 1*59 was compared with that at n — l'<dl. 



General Results. 



A large number of observations were made and the mean 

 taken, the result being shown in the curve given in fig. 4. 

 The method of construction of the curve will be understood 



