122 Mr. W. J. Lewis's Crystalhgmphk Notes. 



The plane a is doubtful. The planes h were only measured 

 on one side of the crystal ; and a, possibly a result of repeti- 

 tions of the opposite k planes, was found on the other side. 

 The difference in the angles to w lt and w ic 3 given by Brooke's 

 data and those adopted by me suffice, I think, to justify my 

 belief in an erroneous impression as to the prism-plane which 

 gave the reading 56° 30'. The angles b 8 and hw are unmis- 

 takable, and are those used in determining the elements ; the 

 remaining angles found by Brogger and myself agree as nearly 

 as can be expected with those calculated. 



Quartz. — In the Cambridge collection are two crystals of 

 quartz, each of which has a well-developed plane whose indices 

 were determined by the late Prof. Miller to be (50,19, 19). 

 Professor Miller seems never to have published this result ; nor 

 has any record of the measurement which led to it been found 

 amongst his papers. I therefore remeasured the crystals, and 

 obtained an angle which agrees almost exactly with that 

 required by the indices (50, 19, 19). The larger crystal is a 

 broken prism about 40 millim. long by 8 millim. across. The 

 face y on this crystal is about 1*5 millim. long by about -75 

 millim. broad, and is smooth and bright. Near the edge \by\ 

 it is slightly rounded. As shown in the diagram (fig. 4), it 

 has to the left a large rough x plane and a long somewhat 

 narrow s plane. The r plane above y is developed so as to 

 all but blot out the other terminal planes, and is traversed by 

 a few horizontal lines, due to repetition of r and some plane 

 in the zone [ry~\. At the extreme top the plane r is more 

 strongly striated, and is penetrated in a perfectly arbitrary 

 way by small crystals of quartz which are ill developed. In 

 the zone \zt^\ are three minute planes, angular measurement 

 of which places them in the position *, /, v.. These planes, 

 however, do not succeed one another in this order, but form 

 re-entrant angles, u being that adjacent to r n I next, and 8 

 last, and adjacent therefore to b n . The plains u and / are 

 strongly striated parallel to the edge [6 /; sJ. The planes 

 observed_on this_crvstal are b (2 11), r (100), c(221), 

 3,(501919), 0(813 8), * = «(4 21),.r(4 2 1),w(G,17,l2) or 

 (5,14,10), / (3 10 6). _ 



The smaller crystal is a slender prism about 13 millim. long. 

 The plane y on it is not so well developed as in the former 

 crystal, and it is considerably more rounded near the edge [by]. 

 The image, however, given by it is quite distinct. The plane 

 8 to the right of y is very largely developed ; and .r and r 

 appear as very narrow planes below it. The crystal is a 

 combination of 6(211), r(10 0), 2(221), y (50 19 19), 

 x (412), v (16 5 8), and a plane near $. 



