I lO 



NATURE 



[July 21, 1923 



geological literature of the world. We cannot find 

 here lists of the publications of geological surveys ; 

 these are indexed under their authors and the districts 

 with which the memoirs deal. Maps separately issued 

 are not regarded as " literature " ; but, under the 

 heading " Maps," there is a very useful list of those 

 included, and often concealed, in printed papers, with 

 indications of tlieir scales. 



In connexion with the mechanism whereby pollen 

 is able to induce hay fever, a correspondent has 

 suggested that possibly the pollen grains in the 

 presence of moisture on the mucous membranes might 

 protrude their pollen tubes ; these might penetrate 



the mucous membrane. A view somewhat similar 

 was developed by Blackley half a century ago in hi^ 

 famous " Experimental researches on the causes and 

 nature of Catarrhus aestivus." He showed that 

 neither the size nor the nature of the covering (i\ 

 the pollen can be the essential cause of hay fever, 

 but from prolonged observations he believed that the 

 moisture on the mucous membrane might cause the 

 pollen to swell and to protrude its tube into a mucous 

 gland. While this might explain some of the initial 

 phenomena in an attack of hay fever, he was strongly 

 of opinion that the obnoxious element of pollen was 

 the granular matter in the centre, a view universally 

 accepted to-day. 



Our Astronomical Column. 



Large Meteor. — In strong twilight on the evening 

 of July II a fine meteor was seen at 9'' 9'" G.M.T., by 

 Mr. E. W. Barlow of Wadhurst, Sussex, who con- 

 sidered the object as bright as Venus at its best. The 

 nucleus was pear-shaped and bluish, and a red train 

 followed it along an arc of about 10°. The duration 

 of flight was 4 seconds and the path from 95 Herculis 

 to 77 Ophiuchi. 



Mr. E. H. Smith of Hanwell, W., also saw the 

 object and describes the path with reference to the 

 stars Altair and Antares. The height of the meteor 

 was about 66-54 miles over the English Channel ; 

 length of path 90 miles, and velocity about 22 miles 

 per second. 



The fireball was also observed by the Astronomer 

 Royal and by Dr. Crommelin at the Royal Observa- 

 tory, Greenwich, and they give the azimuth of the 

 end point as 15° W. of south. 



The Spirai, Nebula as Dust-Clouds. — Mr. J. H. 

 Reynolds discusses in Mon. Not. R.A.S. for May 

 the recent suggestion of Prof. Lindemann that the 

 spirals are dust-clouds expelled from the Galactic 

 system by radiation pressure, and shining by reflected 

 starlight. He gives a diagram of the distribution 

 of the spirals, and of their radial velocities, deter- 

 mined by Prof. Slipher. The data as regards size, 

 inclination, and radial velocity appear to fit in fairly 

 well with Lindemann's theory, if one adopts, the 

 eccentric position of the sun in the Galaxy, as given 

 by Prof. Shapley's determination of the distances 

 of globular clusters. The spirals nearest to the 

 Galactic centre would have the highest velocities, 

 but would appear small to us, owing to distance. 

 Those nearest to us would appear large, but would 

 have small radial velocities, their motion being 

 nearly across the line of sight. There is one feature 

 of the spirals, however, that Mr. Reynolds regards 

 as negativing the theory of their shining by reflected 

 starlight. This is the dark absorption stripe which 

 is seen to cross the centre of many of the spirals 

 that are seen nearly edgewise. On the reflection 

 hypothesis, this should be bright and not dark ; 

 its presence seems to prove that the illumination 

 of the spirals comes from within them. Some years 

 ago Mr. Reynolds put forward the view that the 

 spirals were shining by reflecting the light of some 

 bright body in their centre, basing this on measures 

 of the relative brightness of different regions. He 

 now repeats this suggestion, and adds that it may be 

 possible for the condensed matter in the middle 

 of the spiral to give a spectrum of type F or G, 

 without being in a stellar state. He quotes in 

 support some recent experiments on the spectra 

 given by exploded wires. 



NO. 2803, VOL. I 12] 



Stars in the Milky Way and at the Galactic 

 Pole. — The Harvard College Circ. No. 242 contains 

 a very interesting comparison of a Milky Way field 

 with one at the South Galactic Pole made by Mr. 

 Solon I. Bailey. The aim of such investigations is 

 not only to give an estimate of the total number of 

 stars that exist or rather can be photographed, but 

 also to form an idea of their distribution in space. 

 Star gauges were made by the Herschels a century 

 ago, but since then studies of the distributions of 

 stars have been completed by Argelander, Seeliger, 

 Pickering, Celoria, Kapteyn, Chapman and Mclotte, 

 and others. It was generally deduced that there 

 were only twice as many stars in the Milky Way 

 as elsewhere up to about magnitude ten, although 

 Herschel's counts gave a maximum of about 20 times 

 as many stars. in the Galaxy as at the Galactic poles. 

 An increasing Galactic concentration with decreasing 

 apparent brightness was shown by Kapteyn, and a 

 similar but less rapid degree of concentration was 

 deduced by Chapman and Melotte. Mr. Bailey 

 discusses photographs taken with the 24-inch Bruce 

 photographic telescope at Arequipa, Peru, the longest 

 exposures showing stars fainter than the nineteenth 

 magnitude. He gives a very instructive table 

 indicating the relation of length of exposure to 

 limiting magnitudes photographed, showing that as 

 the stars become fainter, longer and longer exposures 

 have to be made to gain every extra magnitude. 

 Selecting a square degree area in Sagittarius in the 

 Galaxy, one of the richest star fields of the Milky 

 Way, and comparing this with the similar area at the 

 South Galactic Pole, he obtains the following counts : 



Number of Stars. 



It will be seen that, up to about magnitude 10, 

 the number of stars in the Milky Way is about 

 2-5 times that at the pole. After magnitude 13, 

 the ratio increases with great rapidity, until at about 

 the 17th magnitude the ratio is more than 100. 

 If the stars were every\vhere as dense as in this 

 part of the Galaxy their number would be 2-5 billions, 

 while if the density were that at the pole, their 

 number would be 23 millions. 



