438 



NATURE 



[June .2, 1921. 



meter embedded in paraffin or celloidin. Since the 

 object moves in a horizontal plane along the arc of 

 a circle, it has the advantage that the sections are 

 flat. The rocking microtome is similar in general 

 construction to the instruments manufactured in 

 previous years, but with improvements in details ; it 

 will cut sections 0002 to 0024 mm. in thickness 

 from paraffin-embedded objects up to about 12 by 

 20 mm. in diameter. The freezing microtome has 

 been specially designed for use in operation work. 

 The thermometers, which give continuous automatic 

 temperature records extending over a considerable 

 period, are of the electrical resistance type, and con- 

 sist essentially of a bulb containing a coil of platinum 

 wire joined by connecting wires to a recorder. The 

 record consists of a series of dots on the chart-paper, 

 impressed every minute or half-minute as is desired. 



Two correspondents have forwarded further sug- 

 gestions for picture-hanging wire in reply to the letter 

 under that title published in Nature of May 19 last. 

 The first relates to the use of single-strand enamelled 

 phosphor-bronze wire of No. 18 B.W.G. This has 



been found satisfactory for pictures of moderate 

 weight. The other method, similar to that described 

 in Nature of May 26, p. 395, is to use ordinary copper 

 bell wire 0055 in. to 002 in. in diameter. Pictures 

 varying in weight from i lb. to 50 lb., using two 

 wires for the heavier pictures, have been hung success- 

 fully with it. The need for straightening the wire 

 carefully is emphasised in both letters, and the advis- 

 ability of avoiding sharp bends at the edges of hooks 

 is mentioned. 



Messrs. W. Heffer and Sons, Ltd., Cambridge, 

 have in the press a work, to be published in three 

 parts, entitled " Dates and Date Cultivation in the 

 'Iraq," by V. H. W. Dowson, of the Agricultural 

 Directorate of Mesopotamia. The three sections 

 comprising the work will deal respectively with the 

 cultivation of Basra date palms and the marketing of 

 the fruit; statistics and details of the experiments 

 from wfiich the average yield of date gardens per 

 acre is adduced ; and the varieties of date palms 

 found at Basra. Part iii., which will be illustrated, 

 is promised for the coming autumn. 



Our Astronomical Column. 



Pons-Winnecke's Comet. — Mr. W. F. Denning 

 writes : — " On May 28 this comet was conspicuously 

 visible in a field-glass. The comet is at present 

 situated in the Milky Way amongst the stars of 

 Cygnus, and is moving to the south-east at the rate 

 of about 35° per day. It is increasing in apparent 

 brightness, and may possibly come within naked-eye 

 vision at the middle of June. The latest observations 

 prove that the comet is farther from the earth than 

 was expected, and that at perihelion it will be about 

 2,000,000 miles outside the terrestrial orbit. Per- 

 turbations by Jupiter in 19 18 have altered the cometary 

 path and lengthened the period of revolution. There 

 may be a meteor shower on the nights from June 27 

 to 30, but the conditions are such that the display 

 may not be a very brilliant one. Observations should 

 be carefully made at the period mentioned, and it is 

 fortunate that the evening sky will be free from moon- 

 light." 



Speculations on the Formation of Spiral 

 Nebula. — M. Alex. Vdronnet contributes an article 

 on this subject to the Cotnptes rendus of the Paris 

 Academy of Sciences for April 18. He examines 

 the effect that would result from the impact of 

 the two components of a binary system. He shows 

 that the energy produced by friction at their surfaces 

 is the most important factor, and that a mass equal 

 to that of Jupiter might produce by impact with the 

 sun a temporary increase of light amounting to twelve 

 magnitudes. Radiation-pressure would then expel 

 the heated particles with high speeds, and the revolu- 

 tionary movement of the stars would give a spiral 

 formation to the scattered particles. The author seeks 

 thus to explain the phenomena both of novae and 

 of spiral nebulae. He concludes that the latter would 

 undergo a rapid evolutionary transformation (in the 

 course of a few centuries). It would seem, however, 

 that the larger spiral nebulae are on too grand a scale 

 to be the product of the impact of a mere pair of stars. 

 The hypothesis is, however, worth considering in re- 

 lation to such nebulae as that which was revealed 

 round Nova Persei bv the light of the outburst, and 

 Hubble 's variable nebula in Monnceros. 



NO. 2692, VOL. 107] 



M. V^ronnet quotes the results obtained by Mr. 

 Van Maanen, from photographs taken at intervals of 

 several years, on the movements going on in certain 

 nebulae. These showed an outward tendency, agree- 

 ing with M. V^ronnet's conclusions. 



The nebula round Nova Persei had evidently been 

 present, though unseen, before the outburst, and the 

 suggestion had already been made that it might be 

 the product of a former impact of the same two 

 bodies that caused the outburst of 190 1. 



The Companion of o Herculis. — In the course of 

 a paper on "Seven Spectroscopic Binaries" {Astro, 

 physical Journal, April) Mr. R. F. Sanford announces 

 that the fainter star of this well-known pair is a 

 spectroscopic binary with a period of 51-6 days, only 

 one spectrum being visible. He further announces 

 that the radial velocity of the centre of gravity is 

 — 37-2 km. /sec, whereas that of the principal star of 

 the visual pair is —32-2 km. /sec. Mr. Sanford con- 

 cludes from this that the visual pair is only an optical 

 one, the components not being physically connected. 

 The following considerations, however, appear to show 

 that this conclusion is unwarranted : — 



The chance of two unconnected stars of magnitudes 

 35 and 5-5 being within 5" of each other and having 

 the same proper motion of 3" per century in the 

 same direction is so small as to be absolutely neg- 

 ligible. Moreover, the assumption of physical con- 

 nection does not involve an unreasonable value of the 

 masses. The spectroscopic parallax of component B 

 is 0018" and the angular separation 4-7", or 260 astro- 

 nomical units (if unforeshortened) ; a joint mass equal 

 to that of the sun would give a relative velocity in a 

 circular orbit of 1-84 km. /sec. The. actual relative 

 velocity is 50 in the line of sight and i-6 at right 

 angles, the position angle having altered by 6° in 

 eighty years. The combined velocity is 5-25, giving a 

 joint mass of (525/ 1-84)% or 8-14 times that of the 

 sun. We know of many greater stellar masses ; for 

 example, in a paper by Herr P. Hiigeler in Astr. 

 Nach., No. 5098, the masses of the components of 

 ti Herculis are calculated to be 7-43 and 2-84 in terms 

 of the sun. 



