QUEKETT MICROSCOPICAL CLUB. 667 



relative retardations (one was exhibited at the lecture), but in 

 determining colours so much depends upon the idiosyncrasy 

 of the observer, and the character of the light, that results 

 can only be relied on within very wide limits. In the smoky 

 atmosphere of a London winter, for instance, the blue of the 

 second order appears to pass into greenish yellow without any 

 definite green intervening. Relative retardation is equal to the 

 product of the thickness of the section and the birefringence, 

 which is the relative retardation in a unit of distance, and is 

 equal to the difference between the refractive indices of the 

 two directions of vibration. In the case of a section of quartz 

 21 microns thick, cut parallel to the optic axis, the indices of 

 refraction are 1*544 and 1*553, and the bi-refringence therefore 

 0*009. Accordingly the relative retardation = 21 x 0*009 = 0*189 

 of a micron. For the purposes of determining the character of 

 extinctions and the amount of relative retardation a quartz 

 wedge or mica ladder may be employed. 



Dr. F. E. Wright, of the Smithsonian Institute, Philadelphia, 

 devised a combination of quartz wedge and gypsum, and Dr. 

 Evans has successfully employed the same idea. 



A quartz wedge is superposed on a gypsum plate, both being 

 constructed with the usual orientation, so as to leave beyond 

 the* thin end of the wedge a square of gypsum, which may be 

 used as an ordinary gypsum plate. The quartz will show a 

 black band where it neutralises the gypsum. The point is 

 marked zero. Every hundred micro-m.m. of relative retarda- 

 tion is shown either way. If the direction of the crystal section 

 parallel to the slot be fast, the band w r ill move towards the thick 

 end of the wedge ; if slow, towards the thin end. 



The mica ladder consists of a succession of narrow cleavage 

 plates of muscovite, with their length cut parallel to the trace 

 of the optic axial plane, and therefore slow. Each strip should 

 have a relative retardation of 100 micro-m.m. They are of 

 different lengths and superposed to form a succession of steps 

 each large enough to cover the whole cone of light in the lower 

 slot, where they are usually employed, though useful in the 

 focus of the eyepiece, if the upper nicol be placed above them. 

 In either case they show a discontinuous series of colours corre- 

 sponding to differences of 100 micro-m.m. If inserted over a 

 section it is easy to show whether the two show additive or 



