618 SCIENTIFIC RECORD FOR 1885. 



retardation of the two polarized pencils which traverse the thin stratum. 

 Anything beyond this is at present only a matter of speculation Only 

 two directions are indicated in which to look for a possible explanation; 

 and these will form the subject of further investigation. {Nature, April, 

 June, July, 1885, xxxi, 565; xxxn, 102, 224.) 



From the researches of Tollens, Landolt, Schmitz, and others it is 

 well known that cane sugar and many other substances have a specific 

 rotatory power which varies continuously when the i^roportion of inactive 

 liquid is increased. For sugar, however, the diminution is very feeble 

 as the concentration increases, even more feeble than for glucose. With 

 dilute solutions containing 1 to 5 per cent, of sugar, Hesse had found 

 [a]jy = 68° about, a number rather higher than is obtained with more 

 concentrated solutions. Tollens, operating upon dilute solutions with 

 a polarimeter of great sensitiveness, has completely confirmed his earlier 

 conclusions, and finds that whatever be the concentration the formula 

 [ar]D= 66-386 + 0-015035^? — 0-0003986^2^ where j? is the weight of sugar 

 in 100 of solution, always represents the variations of rotatory ]io wer. It 

 has been confirmed on solutions containing from 1 to 67 per cent, of 

 sugar. {Ber. Berl. Ghem. Ges., xvii, 1751.) 



Madan has described a modification of Foucault's and of Ahrens's 

 polarizing prisms. Ahrens's prism Is made of three wedges of Iceland 

 spar cemented together by Canada balsam. The optic axis in the two 

 outer wedges is parallel to the refracting edge, while in the middle 

 wedge it is perpendicular to the refracting edge and lies in a plane bi- 

 secting the refracting angle. By using three prisms the middle one 

 may be given a very large angle, and yet the deviation of the rays m.ay 

 be so far corrected that on emergence they make equal angles with the 

 central line. Nearly in contact with one of the terminal faces of the 

 prism a prism of dense glass is placed of such an angle that it just cor- 

 rects the deviation of one of the rays, and also achromatizes it, while 

 it increases the deviation of the other ray to such an extent that it may 

 be practically disregarded, an eye even when very close to the prism 

 perceiving only the direct beam. {Phil. Mag., January, 1885, V, xix, 69.) 

 Madan finds that the ordinary ray falls on the second surface of the 

 middle prism of this combination at an angle greater than the critical 

 angle and is therefore totally reflected if an air film be placed between 

 this surface and the following one. Moreover, he finds that the devia- 

 tion and the dispersion can be almost entirely corrected by making the 

 third prism of crown glass combined with a prism of very dense flint 

 glass of smaller angle. Its field is 28°. {Nature, February, 1885, xxxi, 

 371.) 



ELECTRICITY. 



1. Magnetism. 



Werner Siemens has published a theory of magnetism and has de- 

 tailed the experiments upon which it is based. In the first place, the 

 Amperian theory must be extended by supposing that not only mag- 



