2bS 



NATURE 



[May 15, 1913 



Diffraction Patterns from Crystals. 



The attention given, in this laboratory and else- 

 where, to the effects produced by passing a beam of 

 Rontgen radiation through crystals suggested to me 

 that it might be of interest to examine the image 

 produced when a narrow pencil of ordinary light 

 falls on a photographic plate after passing through 

 a crystal. The lens was removed from a camera, 

 and in place of it there was attached a tube about 

 30 cm. in length and 3 cm. in diameter. The tube 

 was lined with black velvet, and provided with three 

 diaphragms pierced with pinholes from one-half to 

 three-quarters of a millimetre in diameter. In this 

 way I endeavoured to secure that a cylindrical pencil 

 of light of small cross-section should enter the 

 camera. In consequence of diffraction at the last 

 aperture the impression on the photographic plate, 

 when no crystal was interposed, sometimes extended 

 over a considerable area, resembling the diffraction 

 images recently discussed by Mr. J. W. Gordon (Proc. 

 Phys. Soc, vol. xxiv., p. 428, 1912). 



The crystal to be investigated was placed at a 

 distance of about 5 cm. from the last aperture, and 

 about 20 cm. from the photographic plate. The re- 

 sulting image often showed lines or streaks radiating 

 from the centre at angles depending on the crystal 

 and its orientation. These effects are obtained only 



Phlogopite. 



by giving a prolonged exposure or using an intense 

 source of light. With a mercury-vapour lamp, five 

 or six hours' exposure was given ; with diffused day- 

 light, several days' exposure was required ; but by 

 employing the direct light of an arc lamp an exposure 

 of five or ten minutes was found sufficient. 



I have only found such radial streaks in cases where 

 the crystal showed a more or less streaky appearance 

 when examined by the naked eye. It seems clear that 

 the striations in the crystal act just like a diffraction 

 grating. In the case of selenite the complete pattern 

 resembles an eight-rayed star; the angles between 

 the radial streaks seem to agree with the angles 

 between the axes a and c and (possibly) the first and 

 second median lines. 



I desire to thank Dr. Sibly and Dr. Tutton, to 

 whom some of the photographs were submitted, for 

 crystallographic information. Dr. Tutton writes : — 



"I should think the phenomenon of the streaks in 

 the photograph is due to the well-known ' asterism.' 

 Asterism is particularly well shown by mica, especi- 

 ally' the variety known as phlogopite. It is due to 

 fine enclosures arranged along the glide-planes, anc j 

 parallel to the cleavage plane. It shows itself as a 

 six-rayed star (or occasionally twelve-rayed) when the 

 mica is held between the eye and a bright source 

 of light. Calcite also shows asterism, certain crystals 

 NO. 2272, VOL. 91] 



held in front of a candle flame showing a radiating 

 star of light. In this case, however, it is due to 

 tubular cavities parallel to the glide-planes (the rhom- 

 bohedron known as e{no}). Some sapphires also 

 show asterism, and here again it is due to tubular 

 cavities in planes parallel to the prism {101}. I am 

 not aware that selenite has been studied for asterism. 

 I should think it is very likely that certain specimens 

 will show it, those rich in cavities or enclosures. 

 The cavities would probably be parallel to the per- 

 fect cleavage plane (the symmetry plane b{oio} along 

 which selenite crystals are tabular), and possibly along 

 the two minor cleavage directions 0(100} and 

 jifTn}, the former of which would be parallel to tin- 

 vertical axis r. They might also lie parallel to the 

 inclined axis a. In this case your phenomenon would 

 be at once explained." 



Although the results obtained do not reveal any new 

 physical phenomenon, I shall be glad if the experi- 

 ments prove of interest to the crystallographer as 

 affording him a convenient method of studying certain 

 features of crvstal structure. H. S. Allen. 



Wheatstone Laboratory, King's College, London. 



Bird Protection and the Collector. 



The protection and preservation of wild birds in 

 Great Britain is in these days beset with difficulties 

 of many kinds; but as regards especially the preserva- 

 tion of the rarer birds of our country, the one great 

 and ominous danger is the individual whom Sir Her- 

 bert Maxwell has described as "the cursed collector." 

 At this time of the year the professional collector of 

 eggs infests the country wherever rare species are 

 known or discovered to breed, and wherever clutches 

 are to be had which have their value in the market. 

 At all seasons of the year the professional collector of 

 birds is despoiling the land of the noble, the beau- 

 tiful, the unfamiliar forms of bird-life that hide in 

 diminished numbers among little-frequented commons 

 and heaths, mountains and lakes, woodlands and 

 forests, or visit the country in small numbers 

 and nest at their peril, with a price upon their 

 heads and upon every egg they lay. 



It may indeed be said that while a percentage of 

 naturalists are working to protect birds, to keep up 

 the number of our native species, and to bring about 

 some comprehension of the living creature, others 

 are eager only to secure for themselves, by hook or 

 by crook, the skins and feathers, and the empty 

 shells, to fill their cabinets or to sell or barter. As 

 one of their number has written, with unconscious 

 irony, the preservation of a bird should begin as soon 

 as it is killed. 



A correspondent of The Times wrote the other day 

 of the egg-collector: — "These pests know no boun- 

 daries and observe no laws which stand between them 

 and the objects of their rapacitv." Could the experi- 

 ences of some of the workers in this society be 

 divulged the truth of that statement might be 

 thoroughly demonstrated. Neither the "rarity" col- 

 lector who values the " British-taken " bird or egg 

 solely because it seldom occurs or is on the verge 

 of extinction in Britain, nor the "rarity" collector to 

 whom the speckles on an eggshell are things of 

 absorbing interest, will stop at anv artifice or any 

 dodge in pursuit of his quest. 



The problem lies in this, that the offenders are 

 largely men of wealth and position, officers in the 

 Army, clergymen, "ornithologists," popularly known 

 for their interest in bird-life, and even for their pro- 

 nouncements in print on bird protection ; and that 

 these collectors not only snap their fingers at the law 

 and take pride in evading and transgressing its pro- 



