Regularity of Structure Oj Actual Crystals. 97 



both sides of the plate. The absence of irregularities must 

 not be concluded too hastily. It will happen occasionally 

 that at a particular angle of incidence there is little differ- 

 ence to be recognized upon the two sides of a line of division 

 which is conspicuous enough at a somewhat different incidence 

 of the light. The reason will be apparent to every one familiar 

 with interference phenomena. 



In some sheets a considerable area appears uniform ; and 

 there were two or three on which no abrupt changes of 

 brightness could be detected however the incidence was 

 varied. One of these was submitted to further observation 

 by the more elaborate method described in ' Scientific Papers/ 

 iv. p. 56, in which provision is made for maintaining constant 

 a small angle of incidence. No differences of brightness 

 could be perceived ; but I am not sure that in dealing with a 

 flexible and not perfectly flat sheet the method is really more 

 searching. The conclusion that I felt justified in drawing 

 was that there is no abrupt change of thickness capable 

 of producing a shift of J- or ^ of an interference-band, and 

 no gradual change giving a shift of say \ of a band. The 

 interpretation is discussed at the close of this note. 



In order to submit the equality of thickness to a further 

 test, I divided the sheet of mica into approximately equal 

 parts, cutting it with a pen-knife guided by a straight-edge. 

 A comparison of the relative areas and relative weights of 

 the two parts would then give material for a comparison of 

 relative thicknesses, or at least of relative densities reckoned 

 superficially. 



The weights (w, w r ) of the two parts distinguished as 

 " plain " and " marked " respectively were easily found, the 

 difference being determined with special care by weighing 

 them against one another. The weight of plain was *2466 gin., 

 and the difference was '00828 gm., so that 



w'-w -00828 aoo . 

 w *24bb 



The comparison of areas was a more difficult matter, if 

 only because the edges were not everywhere well defined. 

 Both pieces were very nearly rectangular in shape, the shorter 

 side (a) of plain being 49*1 mm., and the longer (b) being 

 58'6 mm. The comparison was effected with the aid of a 

 reading microscope, which was used to measure, not the whole 

 width, but always the difference. For this purpose the two 

 pieces were approximately superposed in such a manner that 

 the edges were nearly parallel but sufficiently separated to 



Phil Mag. S. 6. Vol. 19. No. 109. Jan. 1910. H 



