88 Wheeler — Experimental Investigation on the Reflection 



expected that the transition layer would have an index less 

 than unity and that consequently according to the theory the 

 phase difference would he increased and the ellipticity decreased 

 over the values they would have if no such layer were present. 

 However, from general considerations as to the degree to which 

 it would he possihle for the two media to interpenetrate and 

 produce a transition layer, it would seem unlikely that we 

 could obtain any evidence as to its existence ; exactly as any 

 such evidence fails in the case of reflection from transparent 

 solids. And in confirmation of this expectation it is found 

 that for all metals, increasing the cleanliness of the surface 

 shows uniformly an increase of the phase difference and a 

 decrease of the ellipticity ; there is never any indication of any 

 reversal of the sign of the effect. So that it is fair to assume 

 that when perfect cleanliness has been attained, the correct 

 values of the phase difference and the ellipticity are yielded by 

 experiment, the effect of the transition layer being negligible. 

 Further light on the question of the transition layer is 

 afforded by experiments on the reflection from metal mirrors 

 immersed in transparent liquids. Here the uncorrected theory 

 leads to an expression for the index of refraction of the 

 liquid {n') in terms of the observed phase differences and 

 ellipticities in air and in the liquid respectively.* An examina- 

 tion of the recorded observations of this nature shows, how- 

 ever, very considerable discrepancies between the values so cal- 

 culated and those given by the direct refraction methods (?i ). 

 In the following table are given the values of n' and n as cal- 

 culated from the experimental results. It includes all of the 

 observations on record.f 



• n' n Observer 



Gold in water 1-30 1*33 Conroy 



" " CS 2 1-54 164 



Silver in water .. 1*25 1*34 Quincke 



" " " 1-25 1-33 Conroy 



" " " 1-43 1-34 Sissingh 



" "turpentine 1*44 1-47 Quincke 



" " CC1 4 1*35 1-46 Conroy 



Copper in water 1*33 1-33 Drude 



" " alcohol 1-32 1-37 " 



" " cassia oil 1*60 1-61 " 



" '< CS 3 1-52 1-63 



Mercury in water. 1*24 1*33 Des Coudres 



" u alcohol 1-28 1-36 " 



* See below, under heading, " Theory." 



f The values are taken from a paper by Drude, Wied. Ann., xxxix, p. 539, 

 1891. The calculations have been verified except in the case of the experi- 

 ments of Sissingh and Des Coudres, to -whose original papers I have not had 

 access. 



