286 Correspondence — C. Davies Shevhorn. 



has not hitherto been recorded from Cornwall. — (2) A protractor for 

 use in constructing stereographic -and gnomonic projections ; by 

 A. Hutchinson. A short historical account was given of the stereo- 

 graphic projection, and a protractor designed to facilitate its con- 

 struction was shown. By the aid of this protractor the radii of both 

 great circles and small circles could be readily determined. It can 

 also be applied to the construction of the gnomonic projection and to 

 measuring the angles between planes and zones. — (3) Supplementary 

 notes on the mineral kaolinite ; by A. B. Dick. Further observations 

 on the optical characters of kaolinite from Anglesea lead to some 

 alterations in the data given in a previous paper. The refractive 

 index is about 1-563 for sodium light, and the optic axial angle, 

 2 Y, is about 68° instead of 90°. The double refraction is very low. 

 Kaolinite from limestone at Hambleton quarry, Bolton Abbey, York- 

 shire, and from sandstone near Newcastle-on-Tyne were described. — 

 (4) An attachment to the goniometer for the measurement of complex 

 lamellated crystals ; by H. L. Bowman. The apparatus, consisting of 

 a small screen pierced by a pin-hole, can be attached to a goniometer, 

 and is capable of adjustment so that minute portions of a crystal face 

 can be successively illuminated. — (5) A new form of quartz-wedge, 

 a modification of the Wright-wedge ; by J. W. Evans. A quartz-wedge 

 cut parallel to t is placed over a gypsum-plate parallel to a showing 

 red of the first order, and extending beyond the thin end of the 

 wedge, so that the projecting pox'tions can be used as an ordinary 

 gypsum-plate. The region where the wedge overlies the gypsum is 

 graduated at the position of exact compensation and at each thousand 

 micromillimetres of relative retardation. If when placed over a 

 mineral in the diagonal position the black band is moved towards the 

 thin end of the wedge, the direction of insertion is that of the 

 vibrations which traverse the mineral with the smaller velocity ; if 

 towards the thick end, the direction is that corresponding to the 

 greater velocity. — (6) Calculation of the chance that the double 

 refraction of a crystal section cut at random shall exceed a particular 

 fraction of the maximum; by H. Hilton. The problem is soluble 

 completely for a uniaxial and partially for a biaxial crystal. 



coE,E,Els:po3^TI^E!^s^OE!. 



ox THE DATE OF PUBLICATION OF FREDERICK DIXON'S 

 " GEOLOGY OF SUSSEX." 



SiE, — It will be remembered that Frederick Dixon died in 1849, 

 leaving his "Geology of Sussex" in manuscript, and that Richard Owen 

 undertook to see it through the press. John Morris, in his " Catalogue 

 of British Fossils," 2nd ed., 1854, notes that Dixon's book was 

 published in 1852 (not 1850 as stated on the title-page), and this 

 supported by rumour led William Davies to make the same statement 

 in some of his letters (MSS. concerning types of " British Fossil 

 Yertebrata," Geol. Dept. Library, Brit. Mus. (N.H.), Pressmark 

 16.0. W., p. 29), in Avhich statement he was not alone. A certain 

 amount of uneasiness has thus arisen regarding this book and the 



