176 Mr. W. J. Lewis on the Analysis 



rotates the plane of polarization in the same direction as the 

 other gases. 



Strasburg, October 1878. 



Postscript. — Since the publication of the foregoing experi- 

 ments we have further improved the apparatus employed, by 

 giving to the iron tube (a a in the figure) a length of 2*4 

 metres. The glass plates d d were now so far from the ends 

 of the six coils that, on closing a current from 70 large Bun- 

 sen elements, they did not produce any perceptible rotation. 

 A repetition of the experiments with bisulphide-of-carbon 

 vapour gave now also an evident rotation of the plane of pola- 

 rization. 



In like manner we succeeded in observing the electromag- 

 netic rotation in gaseous sulphurous acid at 100° C. and a 

 pressure of about twenty atmospheres, and in sulphuretted 

 hydrogen gas at the ordinary temperature and about twenty 

 atmospheres. 



In air, up to twenty-five atmospheres, we have not yet been 

 able to observe any rotation. We will not omit to remark 

 that, apart from employing very high pressures, another way 

 presents itself of making the observations for the investigation 

 of the rotation in air — with polarized sunlight, which with 

 the aid of heliotropes in the direction of the earth's magnetic 

 meridian is sent through a long stretch of the atmosphere. 



XXXI. Note on the Analysis of the Rhombohedral System. 

 By W. J. Lewis, M.A., Felloic of Oriel College, Oxford*. 



HPHE methods followed by Professor Miller and most writers 

 A for obtaining the formulae employed in determining the 

 indices of a form in the rhombohedral system from the mea- 

 sured angles, or, conversely, the angles from the given indices, 

 are, though elegant, difficult and perplexing. It occurred to 

 me that they might be easily obtained by means of the anhar- 

 monic ratio of four poles in a zone applied to three known 

 poles in one of the planes of symmetry, and a fourth pole 

 whose position and indices can be directly connected with the 

 poles of the form to be determined. This method brings out 

 in a prominent manner the relation (2); a relation to be found 

 in all the books, but so disguised and so little noticed as easily 

 to be passed over, whereas from its simplicity, and from the 

 fact that the angle involved in it is the first deduced from the 



* Communicated by the Crystallological Society. Eead Nov. 24, 



1878. 



