of Crystalline Films. 303 



would give us from the second to the fourth orders, and so 

 on. I have not here the plates to show this in detail ; but I 

 have brought a thick plate of selenite, not measured, but 

 hurriedly mounted for this afternoon. We throw its spectrum 

 on the screen first : from the seven or eight dark bands it 

 appears to be from eight to ten waves in thickness; it is at all 

 events so thick as to show no colour. But now superposing 

 the wedge, the shifting of the bands shows the precise compo- 

 sition for every successive \ \ of retardation even in this high 

 order of interferences. It is all rendered by spectrum analysis. 

 Another great use of such a wedge is for gauging the thick- 

 ness of films in making other preparations, for which I use it 

 constantly: we only have to superpose the film to be gauged 

 with its axis crossing that of the wedge, and the stripe that is 

 nearest extinction when the Mcol is crossed gives the thickness. 



The rotatory colours of films are also beautifully shown by 

 mica preparations. We all know that if a film \\ thick (the 

 terms " thick " or " thickness " of course mean in retardation 

 of the slowest ray, throughout this paper) is adjusted with its 

 polarizing planes at 45° with the plane of polarization, we 

 obtain a single circular vibration. But if we adjust in this 

 position a film giving colour next the polarizer, and introduce 

 after that the \\ plate, with its planes at an angle of 45° with 

 those of the colour-film, both the two rays which emerge 

 from the first film are converted into rays circularly polarized, 

 but in opposite directions ; and hence we get approximately 

 the rotatory colours of quartz as the analyzer is rotated. The 

 geometrical figure I now insert is thus circularly polarized, 

 and will illustrate not only the beautiful rotational phenomena 

 of the colours, but also that superior delicacy and intensity of 

 these lower-order colours which has been alluded to : it would 

 be exceedingly difficult to get colours like these in selenite. 

 Again, we take the 24-section wedge used just now, and su- 

 perpose upon it a \\ plate made in two halves, one of which 

 has its planes reversed as compared with the other; on rota- 

 ting the analyzer the colours appear to pass along the wedge in 

 opposite directions, as if it were made in two halves of right- 

 and left-handed quartz. 



My friend the Rev. P. R. Sleeman lately suggested to me 

 another preparation, which was in turn suggested to him by 

 a beautiful one in quartz belonging to the President of the 

 Royal Society. This is a quarter-wave plate divided into 

 twelve sectors. In the position now on the screen the polari- 

 zing planes are all perpendicular and horizontal: but the prin- 

 cipal plane or " axis " is reversed (as in fig. 4) in every alter- 

 nate sector. If we superpose this upon a mica-film giving 



Z2 



