May 22, 1913] 



NATURE 



295 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Reflection of X-Rays and Related Phenomena. 



In a letter to Nature of April 17, M. de Broglie 

 described bands or fringes observed in the photographs 

 produced by reflection of X-rays on certain crystals. 

 Further experiments seem to show that there are two 

 or even three different types of bands present, which 

 must be attributed to different causes. 



First, there is the ordinary dispersion, with the 

 difference that in the case of a three-dimensional grat- 

 ing the spectrum of the primary beam, presumably 

 continuous within certain limits, will appear as a 

 series of bands as certain wave-lengths are destroyed 

 by interference. This would lead to an apparently 

 abnormal lengthening of the spots at a distance from 

 the centre, which has, in fact, been observed. It 

 would not, however, account for bands in the principal 

 regularly reflected spot. 



Secondly, the bands of interference described by 

 Hupka and Steinhaus must be present whenever the 

 primary beam is divergent. As was to be expected, 

 these appear to be present in all the spots if the focus 

 of the kathode rays on the anti-kathode is sufficiently 

 small. They seem to indicate the existence of X-rays 

 of considerably shorter wave-length than the average 

 wave-length in the primary beam, and may possibly 

 be due to fluorescent radiation. 



Thirdly, very strongly marked bands are often ob- 

 served, which must probably be attributed to invisible 

 cracks along the planes of cleavage in the crystal. 

 In certain circumstances the movement of the focus 

 of the kathode rays in consequence of changes of 

 hardness in the tube may enhance this effect. The 

 fact that the bands are nearly equidistant in a large 

 number of different crystals of the same substance 

 might possibly be attributed to the varying velocity of 

 growth of the crystal on account of the seasonal 

 changes during its formation. 



As M. de Broglie pointed out, analogous figures to 

 those obtained by photographing the reflection of 

 X-rays on cubic crystals may be produced by reflection 

 of visible light on a square crossed grating. Laue's 

 theory, which seems to be equivalent to Bragg's, if 

 one assumes cubical packing, shows that only a limited 

 number of lines of definite wave-lengths appear on 

 the plate if one has a three-dimensional grating or 

 space-lattice. Reflection on some crystals, e.g. the 

 base of a prism of phosgenite, (PbCl)_.CO,, appears 

 to show all the spots a two-dimensional grating would 

 lead one to expect, i.e. only the surface layer appears 

 to come into play. Whether this is due to its opacity 

 to X-rays or to the fact that the mean distance apart 

 of the atoms in the direction vertical to the reflecting 

 plane may be an irrational fraction of the distance in 

 the reflecting plane, has yet to be investigated. If, 

 however, one accepts the hypothesis that we have here 

 reflection on the surface layer only — an hypothesis 

 which the number and position of the spots would 

 seem to justifv — then we have in this case true spectra 

 of the X-ravs emitted by the tube and not, as in Laue's 

 experiments, X-ravs of definite wave-lengths sorted out 

 by the grating. The spectra appear to comprise about 

 one octave with a mean wave-length of x = o-037c, 

 where c is the distance of two neighbouring reflecting 

 atoms. It appears difficult to obtain good photographs 

 with this crvstal, as with most others containing 

 elements of high atomic weight. This may be due to 

 NO. 2273, VOL. 91] 



the increased amount of secondary fluorescent radia- 

 tion and to the greater sensitiveness of the photo- 

 graphic plate to these rays. 



The examination of a series of crystals of the regu- 

 lar system confirmed the consequence of all theories 

 and the experiments of various physicists, that the 

 figures obtained must depend only on the position of 

 the plate and the crystal with respect to the primary 

 beam. It is difficult to give definite data as to the 

 reflecting power of different crystals, though it seems 

 that it may be taken as a general rule that those com- 

 posed of elements of lower atomic weight reflect better 

 than those containing heavier atoms. The relative 

 intensity of different spots varies in different crystals, 

 probably according to the distribution of energy 

 amongst the different wave-lengths in the primary 

 beam. But even in one and the same crystal the 

 intensity of different spots varies according to its 

 position. Thus with an ordinary square crossed grat- 

 ing the spectra are at the points of intersection of a 

 series of concentric circles and hyperbola;. By turning 

 the grating in its own plane by 45 the circles open 

 out into hyperbola; and vice-versd. When the plane of 

 incidence is parallel to the lines in the grating spots 

 of equal brilliance are on the circles ; at 45 the same 

 spots, still of equal brightness, are on the hyperbolae. 

 The same experiment carried out with X-rays reflected 

 on rock-salt shows that the spots of approximately 

 equal brightness are on the circles. 



M. de Brogue. 

 F. A. Lindemann. 



Stratigraphical Problems in New Zealand. 



Though I do not in any way object to the review 

 of my book on the geology of New Zealand pub- 

 lished in Nature (January 30, p. 591), I should like 

 to explain further one or two points, for, from the 

 manner in which they are quoted in the review, they 

 are obviously open to misapprehension. 



It is stated that " it is hard to comprehend why un- 

 conformity should be demanded as a proof of the dis- 

 tinction between two successive geological systems." 

 The fact is that those who have wished to split the 

 system of our younger rocks into distinct parts have 

 insisted upon the existence of unconformities. Care- 

 ful work has, I think, now shown conclusively that 

 such breaks do not occur in these rocks. It is there- 

 fore the wish of some of us to represent these rocks 

 as in fact they are : a simple conformable sequence. 

 The lithological nature of all the lower members 

 shows that they were deposited during a uniform and 

 continuous movement of depression. 



It is true that the lowest members of this sequence 

 contain Cretaceous fossils. These Cretaceous sedi- 

 ments are followed by a considerable thickness (500 to 

 2000 ft. in different sections) of unfossiliferous rocks. 

 Cainozoic fossils then begin to appear — in small num- 

 bers at first — but soon a luxuriant Miocene molluscan 

 fauna is developed. It is, however, well to bear in 

 mind, as is frequently mentioned by Hutton, that 

 several of the genera appear in the Eocene sediments 

 of Australia. Associated with the "Miocene" mol- 

 lusca is an echinoid fauna consisting of thirty-two 

 members, which, in a critical article by Tate, is said 

 to be Eocene with a Cretaceous complexion ; at any 

 rate, all the members of it are extinct. 



The point on which I wish to insist is this. All 

 the lower members of this conformable sequence were 

 deposited during the continuance of uniform physical 

 conditions and "in direct continuous succession. Some 

 time after the Cainozoic fauna had appeared elevation 

 commenced. A series of rocks deposited under such 

 conditions should surely constitute a geological system 



