1889.] on Iridescent Crystals. 449 



upon tlio flame.* But the waves reflected from the muslin arrive in 

 the effective direction, and if of sufficient intensity induce flaring. 

 The experiment consists in showing that the action depends upon the 

 distance between the discs. If the distance be such that the waves 

 reflected from the several discs co-operate, f the flame flares, but for 

 intermediate adjustments recovers its equilibrium. For full success 

 it is necessary that the reflective power of a single disc be neither too 

 great nor too small. A somewhat open fabric appears suitable. 



It was shown by Brewster that certain natural specimens of 

 Iceland spar are traversed by thin twin strata. A convergent beam, 

 reflected at a nearly grazing incidence from the twin planes, depicts 

 upon the screen an arc of light, which is interrupted by a dark spot 

 corresponding to the plane of symmetry. [ Shown.] A similar experi- 

 ment may be made with small rhombs in which twin layers have 

 been developed by mechanical force after the manner of Keusch. 



The light reflected from fiery opals has been shown by Crookes to 

 possess in many cases a high degree of purity, rivalling in this respect 

 the reflection from chlorate of potash. The explanation is to be 

 sought in a periodic stratified structure. But the other features 

 differ widely in the two cases. There is here no semicircular evanes- 

 cence, as the specimen is rotated in azimuth. On the contrary, the 

 coloured light transmitted perpendicularly through a thin plate of 

 opal undergoes no change when the gem is turned round in its own 

 plane. Ihis appears to prove tliat the alternate states are not related 

 to one another as twin crystals. More probably the alternate strata 

 are of air, as in decomposed glass. The brilliancy of opals is said 

 to be readily affected by atmos2>heric conditions. 



* See 'Proc. Eoy. Inst.' Jan. 1888. 



t If the reflection were perpendicular, the interval between successive discs 

 would be equal to the half wave-length, or to some multiple of this. 



