388 SCIENCE PROGRESS 



and the distance apart of the cleavage planes in the mica. No 

 reflection has yet been obtained with an angle of incidence less 

 than 75 but there is no reason to suppose that this means any- 

 thing more than that the time of exposure was not long enough 

 for smaller angles. 



When a somewhat longer exposure (30 minutes) is given to 

 the plate, subsidiary spots appear in a very characteristic manner. 

 In all the crystallographs taken with crystals of any system set 

 in any way, a certain feature of the arrangement of the spots can 

 be traced. There are generally several series of spots forming 

 well-marked ellipses passing through the big central spot. 

 These ellipses are nearly circular : they are in fact sections by 

 the photographic plate of circular cones which have the incident 

 pencil as one generator. The reason for their appearance is as 

 follows. The atoms of the crystal may be classed as having 

 their centres on parallel straight lines as well as in parallel 

 planes. Consider the crystal as divided in this way into a set 

 of parallel rows of atoms inclined at an angle to the direction of 

 the incident radiation. As an incident pulse passes over the 

 successive atoms of any one row, wavelets are emitted from each 

 atom in turn at equal intervals of time and these wavelets will 

 be all in phase in any direction lying on a circular cone having 

 the row of atoms as axis and the direction of the incident radia- 

 tion as one generator. In all these directions at least one con- 

 dition for interference is satisfied, so that the ellipse in which 

 the circular cone cuts the photographic plate gives a locus of 

 possible positions of interference maxima. The ellipses which 

 are so apparent in the crystallographs correspond in this way to 

 densely packed rows of atoms in the crystal. At the point where 

 two ellipses intersect, two conditions for interference are satisfied ; 

 the third is satisfied by the wave length and therefore a spot is 

 to be expected there. This effect is very apparent when the 

 beam is reflected from a slip of mica and a somewhat long 

 exposure given to the photographic plate. As well as the main 

 reflected spot, there are many others reflected from subsidiary 

 planes in the crystal. The greater number of these are arranged 

 on two ellipses which pass through the central spot and intersect 

 in the main reflection. They seem to correspond to a lattice 

 arrangement of atoms in the cleavage plane of the mica, the 

 atoms being at the intersections of two sets of parallel straight 

 lines. The atoms are therefore in rows in these two directions, 



