1226 Journal of Applied Microscopy 



(this Goldscmidt calls the angle point) as follows, see Fig.: Let a a' be the zone. 

 Draw the diameters ^/^/ parallel to <7 a' and/ c perpendicular to a a'. From / 

 with radius/^/ describe an arc. The intersection w with/r is the desired pole 

 of the zone a a' and the angle a 7v a' is the true angle between a and a', and if 

 J- is a point of symmetry, then a 7v s must equal a' iv s. 



In most cases the opposite faces are equivalent faces ; sometimes, however, as 

 in case of positive and negative tetrahedra, they are not equivalent. If the poles 

 equidistant from the point s are non-equivalent, s is the pole of a plane of sym- 

 metry, but if the equidistant poles are equivalent s is the rotation point. 



If no point of symmetry is found the group is not a twinning. 



If s is known it may be made the pole face and the measurements so obtained 

 will give corresponding points equidistant from j- in the projection. a. j. m. 



Stober, F. Sur un precede pour tailler des On account of the objections to the 

 grains mineraux en lames minces. Bull. opaque cements of Thoulet and Mann, 

 Soc. Min. 22:61,1899. , , ^11, r 



the author uses Canada balsam for 



cement and describes a quick, convenient way of mounting and grinding the 



grains, so as to obtain thin sections. l. mci. l. 



Wallerant, F. Perfectionnement au refracto- Author describes apparatus as modi- 

 metre pour les cristaux microscopiques. 

 Bull. Soc. Min. 22: 67, 1899. fied by Czapski. 



Mallet, F.R. On Langbeinite from the Punjab Author concludes that the potassio- 

 Salt Range. Min. Mag. 12: 159, 1899. magnesian deposit, at the Mayo mines, 



consists of langbeinite (2 Mg. So^. K2SO4), intimately mixed with kieserite,. 

 picromerite and epsomite. l. mci. l. 



Fletcher, L. On a Mass of Meteoric Iron from This is the first meteorite recorded 

 the neighbourhood of Caperr, Rio Senguerr, found in Patagonia, and its latitude 

 Patagonia. Min. Mag. 12 : 167,1899. . , ^ , , , 1 r 



IS the furthest south recorded for 



meteoric iron. l. mci. l. 



Hillebrand, M. F. Mineralogical Notes. Melon- The Melonite (/) gives formula Ni Te.,, 

 ite (?), Coloradoite, Petzite, Hessite. Am. y^^^^ ^^^ ^^^^ physical characters and 

 Jour. Sci. iv. 8 : 295, 1 899. . ,, ■,•,-, ^ , , 



IS supposed to be identical with Genth s 



melonite (NijTe.j) from the same source in California. L. mci. l. 



Medical Notes. 



A Point in the Technique of Blood Counting. — I noticed an article in the 

 December number of the Journal, in which the complaint is made that the 

 cross lines in the Thoma-Zeiss Blood Count Apparatus are indistinct under the 

 microscope. Will you allow me to make a suggestion, which is very simple, but 

 which I have found to obviate this difficulty entirely ? It is to lower the Abbe 

 Condenser far below the usual position for using it, until the lines stand out 

 distinctly. Possibly every one has discovered this for himself, but as I have not 

 seen it mentioned, and it took me some time to formulate it into a rule for 

 myself, I hope some one may be helped by the hint. 

 Cortland, N. Y. • Dr. F. W. HiggiNS. 



