of Crystalline Reflexion described by Prof. Stokes. 261 



separate. "When the alteration of wave-length is such that 

 the phases of the reflected waves range over a complete cycle, 

 the resultant vanishes, and a dark band appears in the 

 spectrum. The same thing occurs whenever the relative re- 

 tardation of the extreme components amounts to a complete 

 number of periods. At points approximately midway between 

 these, the resultant is a maximum, but the values of the 

 successive maxima diminish*. Near the central band, where 

 (when the number of alternations is great) a considerable 

 fraction of the incident light is reflected by the system of 

 layers, this way of regarding the matter may cease to be ap- 

 plicable, for then the anterior and the posterior layers act under 

 sensibly different conditions. 



Apart from the magnitude of the complete linear period, 

 something will depend upon the manner in which it is divided 

 between the twins. The most favourable, as it is also perhaps 

 the most probable, arrangement is that in which the thick- 

 nesses are equal. In that case every partial reflexion may 

 agree in phase. If the thicknesses, though regular, are 

 unequal, we may first form the resultant for contiguous pairs, 

 and then consider the manner in which the partial resultants 

 ao;orecrate. 



It will be seen that even if the thicknesses of the twins 

 are equal, there are still two ways in which a regularly lami- 

 nated crystal may vary, as compared with the single kind of 

 variation open to a simple twin stratum. These are the 

 magnitude of the linear period, and the number of periods. 

 Comparison of a number of coloured crystals f seems to 

 favour the view that there are important differences of con- 

 stitution, even when the colour is the same at a given in- 

 cidence. 



In many cases the appearances are such as to suggest that 

 the periodicity is imperfect. A little irregularity might alter 

 or obliterate the subordinate bands, while leaving the central 

 band practically unaffected. Sometimes there is evidence of 

 two or more distinct periods, each sustained through a number 

 of alternations. If the period were subject to a gradual 

 change, the central band in the spectrum of the reflected light 

 wouid be diffused, even at small angles of incidence. The 



* The case is similar to that of the distribution of brightness in the 

 neighbourhood of a %i principal maximum," when light of given wave- 

 length is diffracted by a grating. 



t Tor a rich collection of such crystals I am indebted to Mr. Muspratt. 

 He informs me that, though the result of a second crystallization from 

 comparatively pure liquid-, the coloured crystals are but rarely found 

 When the chlorate is produced by the magnesium process. 



