'222 Mr. W. B. Cartmel on the Anomalous 



For instance, when there was a film of fiichsin whose thick- 

 ness was about a fourth of a wave-length of red light, the 

 bands of one set were displaced with respect to the other set 

 by half a band in the red, and this displacement regularly 

 increased, till a point in the blue-green was reached where it 

 was a whole band. At this point there was a sudden change 

 of half a band and then a regular change took place going 

 from short to shorter wave-lengths, till in the violet there 

 was again a whole band displacement. 



This brings up the question of phase-change. There was 

 evidently about half a band phase-change in the blue-green, 

 but in the violet end the red fnchsin reflects like a transparent 

 body, and it is likely that the difference of phase- change 

 between it and the glass is zero. Pfliiger has gone into this 

 point very thoroughly, and says that from the experiments of 

 Wernicke it is safe to assume that the phase-change by 

 reflexion from fuchsin is the same as that from glass for 

 wave-lengths longer than 640/x/Lt. The measurements were 

 therefore made with the light comprised between the B and C 

 lines. Sunlight was used and a number of gelatine screens 

 interposed at S, fig. 4, so as to keep out light of those wave- 

 lengths not needed, because the films bleach easily. 



The thickness of the fuchsin may be determined from the 

 two sets of interference-bands in several ways, but the method 

 involved in the following formulae, due to Wiener, was the 

 one used. 



We have 2t = mX^^^ = (^m-{-l)Xrn+i, 



where t is the thickness of the film between the two pieces of 

 glass, Xj,i the wave-length corresponding to the centre of one 

 band, and \m+i that of the next band towards the violet. 

 Taking one of the bands due to the film of air between the 

 glass and the fuchsin, which falls between the mth and 

 (m + l)th of the other bands, and calling it V^ we have 



where t' is the thickness of the film of air corresponding to 

 this case. From these two equations follow at once 



t-t': 



^m+1 



m— \/! + l 2 



which gives the thickness of the fuchsin {t — t') directly. 



The thickness of the air-films was regulated so that about 

 five or six interference -bands fell between the B and C lines. 

 Thus there were several sets of bands upon which independent 

 measurements could be made. 



