Prof. Miller's Crystallugraphic Notices. 517 



The faces c, d, e, and the faces parallel to them, constitute 

 the hemihedral form with parallel faces tt 1 5 6. 



The form 15 6 was observed by Haidinger in a group of cry- 

 stals of quartz from Chamonix in IMr. Allan's cabinet, now in 

 the possession of Mr. Greg. The faces of this form are marked 

 d in fig. 147, plate 27, of the second volume of Haidinger's 

 translation of Mohs. In the figure of this crystal the faces d 

 are represented as occurring on the alternate angles only of the 

 six-sided prism; an arrangement which does not accord with 

 the laws of symmetry that prevail in crystals of the rhombohe- 

 dral system. By the kindness of Mr. Greg, I have been per- 

 mitted to examine this group of crystals, and have ascertained 

 that a pair of faces d exists on each of two adjacent edges, in one 

 of the crystals of the group, but the faces are so narrow as to be 

 scarcely perceptible. By using for the bright signal a large 

 aperture in a screen through which sunlight was reflected from 

 a plane mirror, the positions of these faces were determined with 

 sufficient accuracy for the purpose of identification. 



On a substitute for the Reflective Goniometer. 

 The methods of measuring the angles of crystals, described by 

 M. Haidinger in the Sitzungsberichte der Mathem.-naturw. 

 Clusse der kais. Akademie der Wissenschaften, vol. xiv. p. 3, and 

 vol. xvii. p. 187, and by M. Casamajor in Silliman's Journal for 

 Sept. 1857, are highly useful in determining the forms of ciy- 

 stals when better instrumental means are not at command. The 

 following description of a substitute for the reflective goniometer, 

 which I have occasionally used and exhibited in my lectures, is 

 offered in the hope that it also may be found serviceable under 

 similar circumstances. It resembles the last-mentioned con- 

 trivance, but allows the crystal to be adjusted more readily, 

 without being more difficult to construct. It consists of a rect- 

 angular parallelopiped of wood ABC, about 9 inches long, 

 2 inches wide, and nearly 1 inch* thick, into which, near one end, 

 is fixed a wire DEF about 0*15 inch 

 in diameter, bent at a right angle at 

 E, so that DE may be about 5-5 inches 

 and EF 1-5 inch long. GHK is a 

 wire of the same diameter bent at a 

 right angle at H, GH being 1*5 inch, 

 and HK 1 inch long. The wires EF, 

 HG fit into two holes bored at right 

 angles to each other in a small cork. 



On the end G of the wire GH is fixed a bit of wood 0'35 inch 

 square, and 0*7 inch long, to the upper end of which the crystal 

 is cemented. 



