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NATURE 



[February 20, 1913' 



The December, 1912, number of Terrestrial Mag- 

 netism contains an account, by Dr. G. E. Hale, of the 

 attempts which have been made at Mount Wilson 

 Solar Observatory to detect the Zeeman effect due to 

 the magnetic field at the sun's surface. An objective 

 of I ft. diameter and 60 ft. focal length forms an image 

 of the sun on the slit of the ys-ft. spectrograph, which 

 to prevent temperature disturbances is mounted in a 

 vertical shaft in the ground. Photographs of the 

 region near the sodium lines in the third order 

 spectrum were taken, and a neat polarising arrange- 

 ment allowed either the red or the violet edge of a 

 broadened line to be photographed. The plates on 

 measurement showed evidence of a positive displace- 

 ment of the lines in the northern and a negative in 

 the southern hemisphere of the sun, the magnitudes 

 reaching their maxima at about 50° north or south 

 latitude. Further observations are being made with 

 the view of fixing the magnitudes of the displacements 

 more accurately. 



With reference to Mr. R. M. Deeley's letter on 

 retinal shadows in Nature of January 30, we have 

 received other letters bearing upon the point. Mr. 

 H. H. Bemrose thinks, probably correctly, that they 

 arc the same as Purkinje's figures. Mr. C. Wel- 

 borne Piper once saw similar branching lines after 

 experimenting with powerful sources of light. In his 

 case they were coloured red against a background 

 of an approximately complementary colour, whilst 

 with the other eye green vessels were seen against 

 a red background. Purkinje's figures are most easily 

 seen with lateral illumination of the dark-adapted eye. 

 Shadows of the retinal vessels are then thrown upon 

 the sentient layer of the retina, the rods and cones. 

 Mr. Piper's observation is of considerable interest. 

 Black print has frequently been seen to look red when 

 viewed in bright illumination. This is undoubtedly 

 due to coloration of the light by blood during trans- 

 mission through the lateral wall of the eyeball. Birk- 

 hoff, however, has shown that erythropsia or red 

 vision may occur after gazing at a brightly illu- 

 minated surface for ten to fifteen minutes when all 

 lateral light is excluded. Rivers holds that erythrop- 

 sia in general is due to blood, the conditions under 

 which it is observed being such as more or less to 

 eliminate the normal red adaptation of the retina. 

 The transient appearances noticed by our correspond- 

 ents may be regarded as Purkinje's figures seen in 

 unfavourable rircumstances owing to the general 

 diffusion of the illumination. 



The February issue of Tlic Chemical World con- 

 tains a reprint of a hitherto unpublished letter from 

 Sir Humphry Davy to Prof. W. T. Brande, who in 

 1.S13 succeeded him as professor of chemistry in the 

 Royal Institution. Written from Idria, it contains a 

 description of a visit to a quicksilver mine containing 

 veins of cinnabar up to a foot in thickness. Davy 

 also records the occurrence, in the great salt mine at 

 Halstadt, of a blue variety of salt ; as this blue colour 

 is now attributed to the action of radium emanation, 

 it is not surprising to read the statement: "I have 

 been again searching in vain for the cause of this 

 extraordinary colour." Both mines, occurring in 

 NO. 2260, VOL. 90] 



bituminous schist, contained dangerous quantities of 

 inflammable air, and gave to Davy the opportunity 

 of introducing his safety lamp. The letter, with two 

 others, is now in the possession of Sir Willifim Tilden. 



.Some interesting views as to osmosis in soils 

 are developed by Dr. C. J. Lynde and Mr. F. W. 

 Bates in two papers published in The Journal of 

 Physical Chemistry (vol. xvi., pp. 758-781). Experi- 

 ments are brought forward to show that a clay soil 

 acts as a semi-permeable membrane, and that by 

 virtue of the osmotic pressure of the solutions of salts 

 in the soil transference of water can be effected. The 

 efficiency of a soil column as a semi-permeable mem- 

 brane increases with its depth, comparatively long 

 columns being necessary to produce the same effects 

 as a perfect semi-permeable membrance. The view 

 that an osmotic movement of water occurs in soils 

 would explain an increased supply of water brought 

 through the subsoil to the surface in the summer 

 months, when the plants actually need more water, 

 and the beneficial results of soil mulching and certain 

 practices in dry farming. 



The January issue of the Journal of the Chemical 

 Society contains an important paper by Mr. T. R. 

 Merton on the photography of absorption spectra. A 

 method has been adopted which resembles those re- 

 cently devised by Dr. Houstoun, of Glasgow, the chief 

 feature of which is that the actual " extinction- 

 ooefficients " are measured instead of merely the thick- 

 ness of solution required to blot out a particular 

 spectrum line in a photograph. By using this method 

 it has been found possible to determine the actual 

 form of the extinction-curves, and in particular the 

 shape of the single absorption band in the visible 

 spectrum of cobalt nitrate (Proc. Chem. Soc, January 

 23, 1913)- It appears that the curve showing the 

 width of spectrum absorbed is of a simple mathe- 

 matical form, its distribution about the central axis 

 of the band being identical with that of the well- 

 known probability curve. The axis of the band is, 

 however, not vertical, i.e. the wave-length of maxi- 

 mum absorption changes slightly with the concen- 

 tration of the solution. It is suggested that the 

 anomalous form of many extinction-curves is due 

 to the superposition of several curves of the above 

 simple form. 



Red Book No. 177 of the British Fire Prevention 

 Committee contains a report on a system of extin- 

 guishing petrol fires which has given very satisfactory 

 results. The system comprises either a permanent 

 installation, wheeled fire appliances, or small extin- 

 guishers, as the case may be, from which certain 

 chemicals are forced, the extinguishing effect being 

 obtained by the combination of two liquids which 

 produce a thick foam which gradually spreads over 

 the surface of the burning petrol, thereby excluding 

 air and extinguishing the fire. According to the 

 report, two petrol fires of considerable area and 

 severity were creditably dealt with, as well as 

 numerous smaller fires, and the extinguishers were 

 also effective on celluloid fires. The system is one 

 which claims the attention of those concerned in the 

 i evcr-inrreasing hazards of petrol, used particularly 



