April io, 19 13] 



NATURE 



'35 



tion there is nothing to show their content. Some 

 of the terms used in insect bionomics might have 

 been included without greatly adding to the bulk 

 of the work; these are not exclusively applicable 

 to insects, but it is in entomological literature that 

 they are chiefly to be met with. It might also 

 have been well to add references in the case of 

 the less usual terms. 



The derivations will be welcomed by many ; 

 they are sometimes omitted, as under "coenogon- 

 ous." Two incompatible derivations are given for 

 •'caterpillar," but the author does not help us to 

 decide between them. A few misprints may be 

 noted; "carneous" for "corneous," under 

 " cranium " ; " unbra " for " umbra " ; " tergum," 

 under "anal angle," probably for "termen," 

 though the latter is insufficiently explained. Other 

 slips occur, but on the whole the book is well 

 suited for its purpose. 



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.] 



Antarctic Barometric Pressure. 



The reduction of the barometric readings taken 

 during the first year of Capt. Scott's Antarctic expedi- 

 tion has shown what I imagine to be an unprecedented 

 rise of barometer from one month to the next. The 

 mean barometer during November was higher than 

 during October by o-Si in. at Cape Evans, o-8o in. at 

 Cape Adare, and 0-87 in. at the Norwegian winter 

 quarters. The rise continued into the next month, 

 and the mean value at all three stations for December 

 was approximately one inch higher than that for 

 October. 



The instability of the atmosphere shown by such a 

 change has a melancholy interest in view of the sad 

 disaster caused by the weather, and is further of great 

 meteorological importance. I should therefore be 

 grateful for any information of similar large changes, 

 so that they may be considered in my discussion of 

 the meteorological results of the expedition. 



The following table gives the mean height of the 

 barometer at the three stations. The data have been 

 reduced to sea-level and normal temperature and 

 gravity. The large difference between the mean 

 values at Framheim and Cape Evans is being inves- 

 tigated. 



Framheim Cape Evans Cnpe Adare 



Simla, March 20. 



NO. 2267, VOL. 91] 



X-Ray Spectra. 



We have recently been carrying out some experi- 

 ments with the object of finding whether spectra of 

 heterogeneous beams of X-rays can be obtained bv 

 letting the rays fall on a crystal surface which would 

 serve as a diffraction grating. 



A beam of rays from a Rontgen bulb was directed 

 on to the cleavage surface of a crystal of selenite at 

 almost grazing incidence, the beam being made prac- 

 tically parallel by means of suitable lead stops. All 

 the photographs taken of the reflected beam show 

 exceedingly well-defined lines, which are not equallv 

 spaced, their number and distances apart varying 

 according to the particular bulb used. These lines are 

 parallel to each other and to the slit. The hardness 

 of the bulb affects the relative intensity of the lines, 

 but apparently makes no difference to their relative 

 positions. Using the same bulb, crystals of different 

 thicknesses all give the same lines. 



The accompanying figure represents diagrammatic- 

 ally the lines obtained in one of the photographs. 

 The direct beam strikes the plate at x, and in the 

 reflected beam are seen three well-defined lines, x lt x 2 , 

 and x s (in addition to what appear to be interference 

 bands, not shown in the figure). When the bulb was 



soft the line x l was very intense, whilst the other two 

 lines were comparatively faint. Another photograph 

 taken with the same bulb after it had been hardened 

 shows the line x, very much less intense than formerly, 

 whilst the lines x 2 and x 3 have increased in intensity. 

 It appears, therefore, that the line .v, is due to the 

 softer constituents of the beam, and the lines x„ and 

 x 3 are due to the harder constituents. That is to say, 

 the rays of longer wave-length are less deviated than 

 the ravs of shorter wave-length. 



The' results suggest that the lines obtained may be 

 spectral lines in the spectra of the beams emitted from 

 the respective 'bulbs. Further experiments are being 

 carried on. E. A. Owen. 



G. G. Blake. 



Teddington, Middlesex, April 7. 



X-Rays and Crystals. 



On repeating the experiments of Laue, Friedrichs, 

 and Knipping on the transmission of X-rays through 

 crystals, I have found that the transmitted rays may 

 easily be made visible by means of an ordinary 

 fluorescent screen, if we use a sufficiently large pencil 

 of rays, and the crystals are sufficiently transparent to 

 the incident ray. 



The X-ray tube used was a Miiller-tube of 20 cm. 

 diameter, with water-cooling ; the current was sup- 

 plied by a Toepler influence machine with sixty plates. 

 The diameter of the pencil of rays was 0-5-1-0 cm. 

 The crystals examined were borax, alum, mica, fluor- 

 spar, rock-salt, rock-crystal, cane-sugar, &c, the 

 thickness varying from 4 mm. to 1 cm. The trans- 

 mitted rays show numerous detached fluorescent spots 

 of elongated shape. If we rotate the crystal about 

 an axis perpendicular to the incident ray, the spots 

 move generally across the central spot caused by the 

 incident ray, but we may choose the axis of rotation 

 such that some of these spots remain stationary while 

 the crystal is rotated. 



Groups of detached pencils are arranged, as it were, 



