December 19, 1912] 



NATURE 



435 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 ccm 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 Rontgen Radiation. 



Following the investigations of Laue, Friedrich, 

 and Knipping, we were led to stud}- the transmission 

 of a narrow pencil of X-rays through rock salt, a 

 crystal of simple cubic form. The results are of 

 interest, for they show in a striking way a strong 

 reflection from the internal cr3'stallographic planes 

 upon which the pencil fell at nearly grazing incidence. 

 The pencil so reflected is of such intensity that the 

 short exposure required to produce well-marked effects 

 on a photographic plate is insufficient to give more 

 than a trace of the most intense of the other pencils 

 of radiation emerging from the crystal. A small cleaved 

 fragment was placed with one pair of faces horizontal. 

 Below this an X-ray tube was fixed to a stand capable 

 of sliding in grooves along an arc of a vertical circle 

 of which the centre was a point in the crystal 

 and the plane was one of the three principal 

 planes of the crystal. A narrow pencil of X-radiation 

 could then be rotated in this plane approximately 

 about the point of incidence on the crystal. 



When the pencil was vertical it was, of course, 

 perpendicular to one face, and parallel to the other 

 two sets of mutually perpendicular principal planes in 

 the crystal. A slight movement of the X-ray tube 

 directed the pencil at nearly grazing incidence on one 

 of the sets of vertical planes. The result was a well- 

 marked spot on the photographic plate situated several 

 centimetres above the crystal, on the same side of 

 these crystallographic planes as the incident pencil. 

 When the latter was made to rotate until it was 

 incident on the other side of these planes, the 

 emergent beam moved through the central direct 

 image to the other side, being again on the same side 

 of the crystallographic planes considered as the inci- 

 dent pencil. The angle turned through by the emerg- 

 ing beam was certainly within a very small possible 

 error the same as that turned through by the incident 

 pencil. We thus have very direct evidence of copious 

 reflection near grazing incidence from cleavage planes 

 within the crystal itself. 



This suggested the probability of a similar reflection 

 from the planes of cleavage of mica, and we proceeded 

 to make a concave mirror of mica to test this. A 

 letter from Mr. W. L. Bragg in Nature of December 

 12, however, announces that this has just been accom- 

 plished. In our experiments with rock-salt the beam 

 enters the crystal in a different manner, but the effect 

 is undoubtedly similar. It is not a pure surface effect, 

 but takes place in the body of the crystal. Whether 

 all the photographic patterns obtainable bv experiments 

 like ^ those of Laue, Friedrich, and Knipping are 

 readily explained by reflection, as suggested by Mr. 

 \V. L. Bragg, our experiments do not yet permit us 

 to say ; but the results of observation of an isolated 

 spot certainly can be accounted for by reflection from 

 a large number of lavers of atoms, parallel to one of 

 the pairs of faces of the crystal. 



Judging from recent experience we have had of 

 the photographic action of X-rays, it appears probable 

 that a beam reflected in such a way is of sufficient 

 intensity to be detected and followed without anv 

 great difficulty by the ionisation method. 



C. G. Barkla. 

 G. H. Martvn. 



King'.s College, London, December 14. 



NO. 2251, VOL. 90] 



Shinobu Hirota. 



Shinobu Hirota, who returns to Japan at the end 

 of this month, by his doctor's advice, came with me 

 ' to this country in 1895, and within a week of his 

 I arrival the seismograph which he brought with him 

 I from Japan was at work at Shide. To convince those 

 who had doubts as to the possibility of recording in 

 Britain earthquakes which had originated even so far 

 away as their antipodes, a second instrument was 

 installed at Carisbrooke Castle. To look after this 

 Hirota had, wet or fine, a daily walk of four miles. 

 The fact that these two instruments gave similar 

 records and also that from a single record we could 

 tell the distance at which a megaseism had originated 

 naturally attracted some attention. Directly it was 

 shown that certain earth disturbances had interrupted 

 cables. Colonies desirous of knowing the cause of 

 these sudden isolations from the rest of the world set 

 up seismographs. 



This was tne commencement of the British Asso- 

 ciation cooperation of seismological stations, now 

 sixty in number. To bring this into being Hirota 

 played an active part. He knows personally many of 

 the directors, and has given instruction to their 

 officers. In practical seismometry he has made many 

 innovations, some of which will perhaps be looked 

 upon as " mere -dodges," but they have rendered in- 

 struments more sensitive. His multiplying levers 

 made of grass stems gathered from "bents" give 

 pointers exactly one-third the weight of their equiva- 

 lent in aluminium, and yet twice, if not three times, 

 as stiff. It was by using these that we got at Bid- 

 ston, where Hirota went to set up an instrument, the 

 first records of rock deformation due to tidal load. 



In the workshop he is a good all-round workman, 

 in the observatory and office he has kept most careful 

 records, could calculate a rhordal distance, make a 

 zenithal projection or an observation for time, while 

 for photographic w-ork he holds a gold medal from 

 the Photographic Society of the Isle of Wight. Above 

 all this, his sharp eyes would find on a seismogram 

 two records where at other stations only one had 

 been discovered. In view of the great attention and 

 large sums which have now been spent, particularly 

 in foreign countries, on the new seismological depar- 

 ture, I "feel myself justified in giving recognition to 

 an assistant pioneer in these new studies. Illnesc 

 carries him back to his native country, where I trusi 

 he will have a speedy recovery. His work is embodied 

 in annual seismological reports of the last seventeen 

 years, and twenty-six circulars giving the records 

 received from observatories cooperating with the 

 British Association. J. Milne. 



Shide, Newport, Isle of Wight. 



The Self-testing of Dispersion Apparatus. 



A serious inconvenience attaches to the standard 

 method of testing a plane grating, echelon, or other 

 dispersive apparatus, by crossing its dispersion w-ith 

 that of an auxiliary piece ; for, unless the resolving 

 power of the auxiliary dispersion is in some degree 

 comparable -with that of the piece to be tested, it is 

 scarcely possible to identify ghosts which lie close to 

 their primaries. When an extended research with 

 crossed dispersions is in question, the case, in most 

 laboratories, becomes even more difficult. 



The difficulty, I think, may be removed by means 

 of a simple and relatively inexpensive arrangement of 

 two front-reflecting mirrors, so devised that the 

 echelon (say) is crossed with its own dispersion. One 

 of the mirror faces has one truly straight edge, at 

 which the dihedral angle is go° or less. This edge 

 is in contact with the face of the second mirror, the 



