86 



NATURE 



[July 21, 1910 



ot organic material, and nitrification. In ottier depart- 

 ments interesting researcli worlc is being pursued actively. 

 Verv complete reports of the work done during the year 

 igoS-c) in each of the scientific departments is included in 

 the volume ; to name a few, that dealing with astronomy 

 and meteorology is by Dr. G. T. Walker, l'".R.S. ; that in 

 geology by Sir Ihomas Holland, F.R.S. ; and in geodosy 

 and geography by Colonel S. G. Burrard, R.E., F.R.S. 

 .An appendi.\ on the economic investigations conducted at 

 the Imperial Institute, by Dr. W. R. Dunstan, F.R.S., 

 completes the volume. 



OVR ASTRONOMICAL COLUMN. 



PiioiOGRAPHS OF AuROR^E. — As an abstract from the 

 Comptes rendus, we have received from the author, M. 

 Carl Stormer, an interesting note on a photographic method 

 of determining the altitudes of aurorse. The difticulty in 

 obtaining such photographs is, of course, the extreme 

 feebleness and the motion of the light, but, by using a 

 cinematograph lens of 25 mm. diameter and 50 mm. focal 

 length, in conjunction with Lumi^re's " violet " plates, M. 

 Slbrmer succeeded in obtaining measurable images, some 

 of which are reproduced in his note. By choosing two 

 stations 4'3 km. apart and arranging for simultaneous 

 exposures, data for determining the altitudes were secured. 

 The four sets of photographs reproduced represent different 

 forms of aurorse seen during March, and also show recog- 

 nisable stars, so that the parallax of definite points is easily 

 calculated. The heights determined are 166 km., between 

 50 and (30 km., 190 km., and 120 km. respectively. 



Displacement of Spectral Lines at the Sun's Limb. — 

 In spectroscopic determinations of the radial velocities of 

 the various solar layers, a difficulty arises from the fact 

 that various perturbations alter the wave-lengths of the 

 lines considered, independently of the rotation. These sub- 

 sidiary displacements have been ascribed to two causes, 

 first, the effect of ascending currents in the solar atmo- 

 sphere, and, secondly, to pressure effects. In order to 

 decide which of these is the disturbing agent, M. A. Perot 

 performed some delicate interferometer experiments which 

 he. describes in No. i (July 4) of the Comptes rendus. In 

 order to determine definitely the e.Kact point of the solar 

 image under observation, M. Perot projected a 36 mm. 

 image on to a copper plate ruled in millimetre squares, and 

 having a circular hole, or a slit, o'l mm. broad at the 

 centre ; thus only the radiations passing through this 

 definite aperture reached his interferometer and spectro- 

 scope. As a result of the experiments, M. Perot deduces, 

 from the form of the curves of relative variation of wave- 

 length obtained, that this relative variation is an effect of 

 pressure, or density, and not of ascending currents. 



The Pressure of Light on Gases.— In No. 5, vol. 

 xxxi., of the Astrophysical Journal, Dr. Lebedew describes 

 a series of very ingenious and delicate e.xperiments by 

 which he has been able to observe the effect of the pressure 

 exerted by a beam of light on various gases. The 

 apparatus is too complex to describe here, but, in effect, it 

 consists of a small chamber in which the gas under 

 examination is contained, and through which a beam of 

 light can be projected in either direction. The pressure 

 exerted by the light produces an excess of pressure in the 

 gas at the farther end of the chamber, and this acts on a 

 very delicate valve which is suspended on one arm of a 

 torsion balance. From a large number of e.xperiments, in 

 which other variable effects were eliminated. Dr. Lebedew 

 succeeded in establishing experimentally the existence of 

 the translatory force exerted by light upon gases, and also 

 ill showing that these forces are directly proportional to 

 the quantity of incident energy and to the absorption 

 coefficients of the gas masses. As these experiments were 

 made, with gases at atmospheric pressure, the numerical 

 values determined cannot be applied directly to such 

 cxcessivclv tenuous masses as are involved in the case of 



comets' tails, but they provide a satisfactory basis on' 

 which further experimental work in this direction may be 

 founded. 



The Determin.^tiox of Stellar Radial \"elocities. — 

 In No. 5, vol. xxxi., of the Astrophysical Journal, Prof. 

 Frost publishes a table of corrections to be applied to the 

 previously published list of radial velocities of certain stars 

 of the Orion type. The corrections are necessitated by the 

 re-determination of the wave-lengths of the three silicon 

 lines at \\ 4553. 4568, and 4575, for which Exner and 

 Haschek's values were previously adopted. Prof. Frost's 

 new measures give 4552'636, 4567'897, and 4574'79i as the 

 correct wave-lengths, and this involves positive corrections 

 of 7'5i km., 3'48 km., and 714 km., respectively, to plates 

 reduced with Exner and Haschek's values. 



As Prof. Frost points out, finality in radial-velocity 

 measures is hardly to be obtained ; the values must be 

 amended as a greater accuracy in the determination of 

 stellar wave-lengths is attained. Further, in the case of 

 blended lines, such as the double helium line at \ 4472, a 

 variation in the relative intensities of the two lines will 

 considerably modify the results. This is especially effective 

 when the adopted blend is uncertain, as in the case of the 

 blending of lines of different elements which may vary 

 considerably from one stellar type to another. Prof. Frost 

 also publishes the data establishing the variable radial- 

 velocity of Rigel, showing a variation from -|- 1 to -|-26km., 

 and states that on one plate a faint component to the line 

 at A 4472 was measured which gave a velocity of — 108 km. ; 

 faint components were suspected in other instances. 



The same journal also contains some further notes by 

 Prof. R. W. Wood on the determination of radial-velocities 

 with objective prisms. After trying several other media for 

 producing fiducial lines, he tried peroxide of chlorine, 

 which, contained in a suitable cell, appears to answer very 

 well. It gives absorption bands which are very well defined 

 on the red edge, and in his 21-foot grating photographs 

 can be measured to within o'02 A.U. ; with such bands, 

 he suggests, radial velocities, of suitable stars, could be 

 determined to within 2 or 3 kms. Unfortunately, the 

 absorption bands cover most of the hydrogen lines, so that 

 this absorbent could not well be used for first-type stars, 

 although it is possible that \ 4863 and \ 3S37 would 

 appear ; for other types a peroxide of chlorine screen 

 apparently answers perfectly. 



Halley's Comet. — .\ number of observations are recorded 

 in .istronomische Kachrichteii, No. 4425, but, in general, 

 they do little more than confirm others previously noted. 



Herr G. Miiller gives an outline of the observations 

 made, at great altitudes, in Teneriffe, and Herr W. Munch 

 describes the general observations at Potsdam. The 

 spectrographic observations made on May 19, from 4h. 55m, 

 to 5h. lom. and from jh. 28m. to 5h. 30m. (M.E.T.), are 

 described by Herr v. d. Pahlen, but no modification of the 

 normal solar spectrum was discovered. The slit covered 

 35' of arc, and two scries of six exposures on the predicted 

 positions of the comet on the sun's disc were made, so that 

 a large area of th" solar surface was covered without 

 revealing any trace of the comet. 



An observ^ation by Dr. Ristenpart, at Santiago de Chile, 

 on July I showed the comet as a nebulous mass i' in 

 diameter, with no condensation, and with a tail 2° long. 



Harvard College Observatory. — In his report for the 

 year ending September 30, 1909, Prof. E. C. Pickering 

 deplores the diminution of 5000 dollars in the income of 

 the Harvard College Observatory, and points out that a 

 disproportionate decrease will have to be expected in the 

 amount of work accomplished. As in previous years, a 

 great number of negatives of stellar regions and stellar 

 spectra were added to the magnificent collection now 

 stored at Harvard, and a number of important discoveries 

 were made from the Draper memorial photographs. 

 Seven meteor trails were found on chart plates, and at 

 Arequipa the spectrum of a very bright meteor was 

 secured. At this southern station the work was sadly 

 upset by unusually had weather, but, among other things, 

 the spectra of more than 400 stars of magnitudes 5-6 

 were secured with the i3-inch Boyden telescope. 



NO. 2125, VOL. 84] 



