1U6 Prof. Reusch on Mica- combinations. 



cipal section coincided with the longer dimension. Let us sup- 

 pose we have prepared forty-eight plates. Half of this number 

 is to be used to pile up a flight of steps, so to speak, ascending 

 in the order 1, 2, 3 from left to right, as shown in fig. 1 ; the 

 other half is to be employed to build up a flight of steps ascend- 

 ing from right to left, as shown in fig. 2. Prior to laying an 

 additional film on the one already iu position, a small drop of 

 thick copal varnish is to be laid upon the latter, and the fresh 

 film is to be pressed down thereon lightly. By this means there 

 are prepared two arrangements or systems, the plates whereof in 

 the one marked R (fig. 1), to an observer who looks at the stairs 

 from the side, ascend towards the right, and in the one marked 

 L (fig. 2) ascend towards the left. Botanists, who in order to 

 determine the direction of a spiral imagine themselves placed in 

 its axis, would, it is true, and perhaps with more propriety, look 

 upon the system marked R as a left-handed one, and the one 

 marked L as being right-handed ; in what follows, however, I 

 shall abide by what is technically and in the language of common 

 life held to be a right-handed and a left-handed screw. 



Now, in the central regular hexagonal portion where the super- 

 position occurs, the systems R and L behave very nearly like a 

 right-handed or left-handed rock-crystal. Even with not more 

 than from four to six turns of mica not excessively thin, the direc- 

 tion of the rotation may be ascertained upon turning the upper 

 NicoPs prism round : in Norremberg's polarizing microscope 

 with a wide field one observes the system of rings with a bluish 

 central cross ; and on employing both arrangements together 

 superposed, very satisfactory indications of Airy's spirals are ma- 

 nifested. 



Precisely similar results are arrived at by the use of two ar- 

 rangements in which four systems of plates are stacked up, form- 

 ing a right-handed and left-handed series crossed at an angle 

 of 45°. 



The arrangements that I constructed in the first instance con- 

 sisted in part of films not very thin, and the thickness of which 

 was not completely uniform; in part, too, the number of turns 

 was but limited (from three to six); and I therefore addressed 

 myself to Mr. Steeg, from whom I speedily received two pairs of 

 systems crossed at an angle of 60°. They are surprisingly large 

 and are executed with extraordinary perfection, and exhibit with 

 brilliancy the change of colour on the upper Nicol being turned. 

 One pair consists, in each combination, of thirty films of ^ X, the 

 other of no less than thirty-six films of even a less thickness. 

 The first pair gives for red light a rotation of 150°, correspond- 

 ing to a thickness of quartz of about 8 millims. As a proof of 

 the great precision and skill with which Mr. Steeg manipulates 



