68(i 



The ineasiiremeiits on pure meroiiry, immediately after tiie formation 

 of the inin-or, gave: 



Angle of Incidence Compensator Angle of Principal Incidence 

 78°38' 56.24 79°J8'. 



Observations made at the same angle of incidence on the mirror 

 with an adsorbed layer of air yielded -. 



78°38' ' 56.90 78°19'. 



It follows from this, that if the compensator-reading in the neigh- 

 bourhood of the angle of principal incidence diminishes by 0.66, the 

 angle of principal incidence increases bj 59', so that it follows from 

 the observed value c,^ — c'o = 0.25, tiiat the adsorbed layer of air 

 decreases the angle of pi-incipal incidence by 22'. 5. It is to be doubted 

 ver^- much whether the change, which has been observed for ci-ystals 

 on natural cleavage surfaces, shortly after the splitting, in the phase 

 difference l)etween the components of the reflected light, which 

 vibrate normal to and in the plane of incidence, should be attributed 

 to layers of air. Drude lound this change for fresh cleavages surfaces 

 of antimony glance, calcspar, and rock-salt. ') For rock-salt the cause 

 is not to be looked for in a lajer of water on the hygroscopical 

 crystal. The observed change in the phase difHerence is greatest for 

 antimony glance at the angle of principal incidence. According as 

 the optical axis of the crystal is parallel to the angle of incidence 

 or normal to it, this change amounts successively to 0.01 and 0.06. 

 This value is many times greater than has been observed for mercury. 

 Besides the greater part of the change has already taken [)lace in 2 

 hours and the retardation in the increment of the ellipticity on standing 

 is much more considerable than for mercury. Drudü considers fresh 

 cleavage-planes as unsaturate and thinks that also a greater conden- 

 sation of the gas layers would have to result from this. Experiment 

 should decide, however, whether gas layers play a part also here. 

 Fresh cleavage-planes of lead glance do not exhibit a change in 

 the elliptical polarisation with the time. 



7. On the changes in the phase-difference obtained in a previous 

 iiwestigation. 



It appears from the observations mentioned in § 5, that changes 

 of nearly 1° in the angle of principal incidence caused by surface-layers 

 were observed. Hence the question is, what gives rise to these 

 greater changes? Very probably these occur in consequence of liquid 

 layers, which are enveloped by the dust falling on the mercury. When 



M Drude, Wied. Ann., 34, 489, 1888; 36, 532, 1889. 



