368 Mr. William Phillips on the Oxyd of Tin. 



direction in fig. 186, and the same proportion of another crystal 

 having been turned half round, and reversed in its direction, is, in 

 that figure, thus attached to it. The incidence of the edge a b on 

 the edge c d, fig. 186, is 112°. 10'.* 



The series of this made, which is the most simple of all in its 

 combination, is described by figs. 203, 204, 205, and 206, 

 PI. 25 ; and as the planes on each are numbered with those of the 

 several modifications to which they respectively belong, they will be 

 readily understood, except perhaps that of fig. 206. This latter, as 

 a reference to fig. 52 will evince, is composed of similar parts of 

 two crystals ; but as the section of the two portions of which it 

 consists took place parallel with a face P of each, which do not ap- 

 pear in the made itself, it follows of course, that this section must 

 be immediately opposed to that of the three preceding figures. The 

 existence of this section will be explained and confirmed in speaking 

 of the formation of the made described by fig. 188, 



Double Macks. 



The made described by fig. 184, PI. 24, may be termed a double 

 made, because it is terminated at each end by a made similar to 

 that described by fig. 191, which resembles that described by fig. 

 186, except that the planes 1, 1, which are those of the prism, 

 are shorter. 



If we were to suppose fig. 187 to consist of two macles similar 

 to fig. 191, simply reversed, it would be obvious that a re-entering 



* The Abbe Haiiy iu his Tableau comparaiif, has given this incidence, as 112°. 16'. 44", 

 but I have been induced to quote it as above, because I have uniformly so obtained it by 

 means of the reflecting goniometer, on macles having the edges « b and c tl replaced by 

 the planes of the fourth modification. 



