370 Intelligence and Miscellaneous Articles. 



the Old Red Sandstone on the south of the South- Wales Coalfield. 

 This was found to be a little over 4000 feet. The thinning-out of 

 the Old Eed Sandstone and Silurian strata, together with the marked 

 change which takes place correspondingly in the lithological cha- 

 racters of the latter formation on passing from the north to the 

 south side of the coalfield were taken to indicate an approach to a 

 shore-line. This shore-line belonged to land which, as shown by the 

 great thickness of the Devonian beds, could not have extended far 

 south. It corresponded to Mr. Etheridge's barrier between the Old- 

 Red-Sandstone and Devonian seas. The sandstones with Old-Red 

 characters, such as the Hangman Grit and the Pickwell-Down 

 Sandstones, occurring in the Devonian formation were deposited at 

 intervals when this barrier was submerged to a greater depth than 

 usual. The Cornstones were stated to thin out to the south along 

 with the other sedimentary beds of the Old Red Sandstone, and 

 were regarded as derived from the denudation of previously up- 

 heaved limestones, such as the Bala and Hirnant. The paper con- 

 cluded with a description of the characters of the more interesting 

 rocks and fossils. 



LX. Intelligence and Miscellaneous Articles. 



ON THE OPTICAL PROPERTIES OF STAKCH. 

 To the Editors of the Philosophical Magazine and Journal. 

 G-entlemek, Vienna, March 31, 1879- 



HPHE January Number of the Philosophical Magazine contains a 

 -*■ paper by Mr. Walter Baily, in which he refers to a former paper 

 of his, "The Optical Properties of Starch," Phil. Mag. for August 

 1876. This paper having at the time escaped my notice, I hope 

 you will now allow me a few remarks on this subject in your 

 Journal. 



In the year 1864 1 published in Poggendorff's Annalen, vol. cxxiii., 

 a note, "Ueber das Kreuz, das gewisse organische Korper im 

 polarisirten Lichte zeigen, und fiber die Haidinger'schen Parben- 

 biischel." I there showed that the cross which starch, the crystal- 

 line lens, sections of horn, &c. give in the polarizing microscope 

 may be explained by a radial structure of doubly refracting mate- 

 rial. 



I proved this also by experiment, by rotating a sector of mica 

 between two Nicol prisms. Using instead of mica a dichroic 

 mineral and only one JN T icol, a cross is obtained the alternate 

 branches of which are differently coloured. In the same paper 

 I made use of this fact for an explanation of Haidinger's brushes. 



At a later period I had a mica disk, formed of sixteen sectors, all 

 identical and cut symmetrically to the optic axes, constructed 

 by M. Steeg, of Homburg. This disk showed the cross very well 

 in the old JMorrenberg apparatus, especially when by a suitable 

 lens the bounding lines between the different sectors were rendered 

 a little indistinct. 



