FUNDAMENTAL FOICIS OF CRYSTALS. 



27 



last system ; but they are oblique to the vertical axes. Their 

 positions are shown in the figures. 



The right rhomboidal prism stands erect when on \t§ rhom* 

 boidal base, as in fig. 11 ; but is oblique when placed on 

 either of the other sides, as in fig. 10. The oblique rhombic 

 prism is shown in a lateral view in fig. 12, and a front view 

 in fig. 13. 



V. The fifth system includes the oblique rhomboidal prism 

 which has the three axes unequal, 14 15 

 and all are oblique in their intersec- 

 tions. Fig. 14 represents a side 

 view of this form, and fig. .15 a 

 front, view. 



VI. The sixth system includes 

 the rhombohedron and hexagonal prism, 



16 16a 17 



TN 



iHA \* R _ 



are 



three equal lateral axes and a vertical axis at right angles 

 with the three. Fig. 16 is an obtuse rhombohedron, and 16a 

 is the same in outline, showing the axes. Figs. 17, 17a, 

 represent an acute rhombohedron. Fig. 18 is a hexagonal 

 prism ; it is bounded by six equal lateral planes ; the lateral 

 axes either connect the centers of opposite faces, as in the 

 figure, or of opposite lateral edges. 



To understand the rhombohedron, the student should have 

 a model before him. On examining it he will find one solid 

 angle made up of three equal plane angles, and another op- 

 posite one of the same kind ; all the other solid angles are 

 different from these. These two solid angles are called the 

 vertical solid angles, and a line drawn from one to the other 

 is the vertical axis of the rhombohedron. The rhombohe- 

 dron should be held with this line vertical ; it is then said to 

 be in position. Thus placed, it will be seen to have six lat- 

 eral angles, six equal lateral edges, and also six equal termi- 

 nal edges, three of the terminal above and three below 



What forms does the fifth system include, and how does this system 

 differ from the preceding ? What does the sixth system include 1 

 What is said of the rhombohedron ? of its position ? its solid angles? 



