ANALYSIS OF CRYSTAL-STRUCTURE BY X-RAYS 389 



clinic cell is shown in Fig. 2 (a). Ignoring for the moment 

 the points P and Q, the fundamental parallelepiped is 

 OBFACDEG, possessing, if OBFACDEG are points or spheres, 

 the full symmetry of the monoclinic system, a plane of sym- 

 metry parallel to the plane OCGA, and a dyad axis of symmetry 

 perpendicular to the plane OCGA. OA, OB, OC are the three 

 axes of reference, OA = a, OB = b, OC = c, while the angle 

 yS = AOC. The angle AOB = 90°. Fig. 2 (a) represents a unit 

 cell of naphthalene, and Fig. 2 (b) of anthracene. 



If now we assume that there is one molecule lying at each 



NAPHTHALENE 



ANTHRACENE 



Fig. 2. — Unit cells of naphthalene and anthracene drawn to the same scale. 



OA = « OB = 6 OC = c 

 Naphthalene 8-34 6-05 8-69 



Anthracene 8-7 6-i ii-6 



Naphthalene x = BOC = 90°, /3 = COA = 122° 49', y = AOB = 90'. 

 Anthracene x = BOC = 90", jS = COA = 124° 24', y = AOB = 90°. 



{By permission of the Physical Society of London.) 



corner of the fundamental cell, and no others whatsoever within 

 the cell, we can easily calculate what the spacings of all the 

 crystal planes should be. But further X-ray examination 

 reveals that the spacings of both the planes parallel to OCDB 

 and those parallel to OCGA are actually half the calculated 

 spacings. These facts lead inevitably to the conclusion that 

 there are two other molecules in the cell and lying somewhere 

 along the line joining the centres of the faces CDEG and OBFA. 

 Bragg places these two molecules actually in these two faces 

 themselves. They are represented in the figure by the points 

 P and O. 



