1883.] Crystallography of Miargyrite. 377 



perceived, and I believe that there are three other planes (12 10), 

 (1 19) and one in the zone [c/3] not already given, to be found on it. 

 In Mr Friedlander's drawing the crystal is represented as hemi- 

 morphously developed, inasmuch as 7 is preponderant on one side of 

 the pole of symmetry, whilst g is so on the other. I do not think 

 this quite the case, and I was most struck by the very large 

 development of one of the planes % on the smaller crystal of the 

 group. Mr Friedlander calculated his elements from angles which 

 involve c, the reflexions from which I do not think very trustworthy 

 as the planes are deeply striated. He also gives the angle 

 ap = 49° 45', a value which on careful examination I am inclined 

 to doubt. I obtained an angle nearly the same as this, but the 

 face a, which gives this value, is intersected by a crack, and one 

 portion has been dislocated from its true position. This dislocated 

 portion is that which gives the brightest image and the value 

 49° 45' ; but that it is not the true face is shown by its being very 

 far from parallel to the opposite a, and also by the image being 

 outside the zones which contain a. The image from the undisturbed 

 portion is less distinct, but it gives ap = 50° 4' 5, 50°J3', whilst the 

 angle between the parallel faces was found to be 50° ll'*5 or 

 50° 10'. He also gives ai — 47° 39'. The angle to the dislocated 

 portion of a was found by me to be 47° 33'; that to the true a 

 47° 40'. Mr H. A. Miers has confirmed my results with the 

 Fuess' goniometer and observed a new plane A (210) lying between 

 s and a. He found ao 55° 5'"5, (50° 8'), 50° 9' ; X X, 83 ° 29 'l 5 

 %% 27° 22 /- 5. This crystal is a combination of 



a, V> d, s, t, co, x, a, i, 0, 7, g, x , k & £ k, A (210), (119) ? (I 2 10)? 



The measurements obtained are given in a column headed "Strass- 

 burg." 



In the accompanying tables I have given the principal angles 

 which I have calculated and observed. The first column contains 

 the calculated values, the other columns give the angles observed 

 on the crystals denoted by the number at the head of the column. 

 Those printed in dark thick type were observed with telescope and 

 collimator, the others with the small vertical goniometer. Where 

 several observations are given on one crystal, they are from corre- 

 sponding faces, unless they are enclosed in brackets, when they 

 are alternative and due to more than one image on one or both 

 faces. 



In Figs. 2, 3, 5, 8, the plane of symmetry is nearly, but not 

 quite, coincident with the plane of the paper. Figs. 6, 9, 10, 14, 15, 

 16, A and A t are orthogonal projections on the plane of symmetry. 

 Figs. 14 and A are moderately close copies of Miller's diagrams of 

 crystals C and A. Figs. 15 and A t are repetitions of these diagrams 



