Prof. Reusch on Mica-combinations. 197 



mica, I may state that the seventy-two films of the second pair, 

 12 millims. in width and 30 milhms. long, were all cut from the 

 same plate. From a thin plate (£X), for which I am indebted 

 to Mr. Steeg's special kindness, I have subsequently prepared 

 systems with four films crossed at 45° which showed Airy's spiral, 

 whereas my original preparations exhibited, it is true, a slight 

 change of colour when the Nicol was turned, but gave when su- 

 perposed a confused image of the rings. 



If a mica-combination of this construction is turned round in 

 its plane in converging light, the planes of polarization being 

 crossed, the rings indeed remain, but the arms of the black cross 

 undergo modifications; and we observe that at the ends of the 

 diameters of the innermost ring, those diameters being situated 

 in the planes of polarization, dark patches enter and withdraw. 

 In like manner, in parallel light, the colour undergoes slight 

 changes on turning the Nicol round, but the modification takes 

 place rather in the intensity than in the tone of the colour. 



I have found that these properties of a mica- combination may 

 be imparted to a quartz by inserting both above and below it an 

 eighth-of-undulation mica film, the principal sections of those 

 films being crossed at right angles. The mica-combinations are 

 consequently to be looked upon as elliptically right and left po- 

 larizing media, which approximate all the closer to quartz the 

 thinner the films are and the greater the number of the turns is. 



In like manner, the modifications of the black cross in Nu- 

 remberg's combination (treated of in paragraph I) may be pro- 

 duced with a plate of calc-spar cut at right angles to the axis 

 when combined in the manner described with eighth-of-undula- 

 tion films, the whole arrangement being turned round in its 

 plane. Hence we see in the mica-combination the phenomenon 

 as though a non-rotating uniaxial crystal were elliptically pola- 

 rized and analyzed. 



3. In forming the layers of the films at an angle of 60°, there 

 are produced on the sides of the central hexagon equilateral tri- 

 angles in which only two systems of films alternately cross at 

 60°. It is easy to convince oneself that in these triangles, ac- 

 cording to their position, one has to do with elliptical right or 

 left rotation. I was thereby led to investigate, as a preliminary 

 step, the conduct of two plates of any thickness whatever, the 

 principal sections of which were crossed at an angle differing from 

 90°. Speaking generally, such a combination gives right or left 

 elliptically polarized light ; that is to say, upon turning the upper 

 Nicol, one arrives at determining the direction of the rotation, but 

 upon turning the combination in its plane, the intensity of the 

 colour is changed, and also its tint as well. The experiment is 

 successful, not only with any two mica or selenite plates, but with 



