NOTES. 



ASTRONOMY. 



By A. C. D. Crommelin, D.Sc. 



DIAMETERS OF STARS.— The question of the diameters 

 of the stars is at once a most interesting and a most difficult one. 

 Both the late Mr. R. A. Proctor and Major Macmahon have 

 suggested using occultations of stars by the Moon for this 

 purpose, the former by putting the star image into rapid 

 rotation and noting the arc over which fading extended, the 

 latter by trying to record the disappearance on a photograph of 

 large scale. However, it is improbable that either of these 

 schemes is workable, and they would be limited to stars near 

 the Ecliptic. M. Charles Nordniann has attacked the problem 

 in another manner, {Coinptcs RcndKs dc VAcadcniw, 1911, 

 January 9th. I It is clear that the problem would be soluble if 

 we knew the distance of a star, its brightness compared with 

 the Sun's at an equal distance and the ratio of its surface 

 brightness to that of the Sun. The first datum is known with 

 tolerable accuracy for the nearer stars ; the second follows 

 from it; the third is the most difficult, but M. Nordmann has 

 given a formula for the " intrinsic effective brightness " based 

 on a study of the relative intensity of different positions of the 

 visible spectrum. (Thus it is generally admitted that the stars 

 with the Sirian type of spectrum have a greater surface 

 bj-illiance than those with the solar type, and still more than 

 those of a decided red colour). This is obviously the most 

 open to doubt of the assumptions, but we may take M. 

 Nordmann's list as giving at least a rough idea of the 

 dimensions of some of our stellar neighbours. He takes the 

 Sun's stellar magnitude as —26-83, which is the result of 

 recent Harvard measures and is in good accord with the 

 mean of the best previous results. 



It will be seen that Aldebaran and ji Andromedae are the 

 giants of the list, the result for the former being entitled to 

 more weight owing to the greater parallax. The above value 

 would make the apparent diameter of Aldebaran 0"- 01 8, which, 

 as M. Nordmann points out, would require an aperture of twenty 

 feet for its direct measurement, .\ direct occultation of this star 

 by the Moon would occupy one-thirtieth of a second, which might 

 be increased two or three times or even more, in an oblique 

 occultation. It will be seen that, according to the figures, 

 Aldebaran surpasses our Sun in size more than our Sun does 

 Jupiter. The likelihood that Sirius does not greatly surpass 

 the sun in size, in spite of its great brilliance, was already 

 inferred from the fact that its mass is only double his, so the 

 new value is quite reasonable. Arcturus. not included in this 

 list, is another great sun ; Miss Agnes Gierke said of it : — 

 " Perhaps the most stupendous orb within our imperfect 

 cognisance." 



THE SYSTEM OF ALG( )L.— Algol, the last star on the list, 

 was the first to have its diameter measured by Vogel using the 

 spectroscopic method to find the rate of motion ; assuming the 

 densities of Algol and the companion to be equal, he found 



1,051.000 miles for the diameter, or 1 • 2 of the -uui as a result, 

 independent of the parallax, and curiously close to that 

 independently found by M. Nordmann. A new and beautiful 

 investigation of this system has recently been published by 

 Mr. Joel Stebbins, of the University of Illinois, {Astrnphys 

 Joiini., October, 1910). He uses a selenium photometer 

 attached to a twelve-inch refractor. An out-of-focus image of 

 the star, seven millimetres in diameter, is thrown for ten 

 seconds on a selenium plate, which is in an electric circuit 

 forming one. arm of a Wheatstone bridge ; the galvanometer 

 deflection is measured, and corresponding readings taken on 

 a Persei and S Persei for comparison. The method is far more 

 sensitive than that of visual comparisons, and reveals (what 

 had long been \dsually sought in \'ain) a secondary minimum 

 when the companion is occulted by Algol, the depression 

 being 0-06 magnitude. He infers that, taking the parallax 

 0"-05, the side of the companion nearest .Algol gives six times 

 the light of the Sun, and the other side three times. The 

 difference is inferred from the fact that a continuous increase 

 in light is noted from principal minimum to secondary minimum, 

 and the probable explanation is the tremendous radiation 

 from Algol to which it is subjected. ( )n different assumptions 

 as to the distribution of density in the system, the estimates of 

 the diameter of Algol range from 0-81 to 1 -45 of the Sun. the 

 companion being one-seventh larger than the primary, and 

 the eclipses partial ; the apparent ellipse being about 8 open. 

 On all the hypotheses the surf.ace brilliancy of .■\lgol is much 

 greater and its density much less than those of the Sun. 



Mr. Stebbins is to be congratulated on his successful appli- 

 cation of the selenium cell, which will doubtless in the future 

 be largely employed for measuring small light changes. The 

 idea is not new, but previous attempts have not met with 

 equal success. One necessary precaution is to enclose the cell 

 in an ice-box, its sensitiveness being greatly increased. 



THE EIGHTH SATELLITE OF JUPITER.— This tiny 

 body will be out of reach at Greenwich for some years, while 

 Jupiter is south of the Equator: it is therefore a matter of 

 satisfaction that it was photographed during February with 

 the large reflector at Helwan in Egypt. The positions are 

 within some 20" of these predicted. The error can be 

 removed by increasing the adopted period (738-9 days) by 

 a quarter of a day, and making a very slight increase in the 

 eccentricity. This satellite will ultimately give a very accurate 

 mass of Jupiter, but it nuist be observed o\'er a larger arc 

 before it can be fully utilised for this purpose. 



ADOPTION OF WESTERN E U R O P I-: A N 

 (GREENWICH) TIME IX FRANCE. — This adoption 

 took effect on March lOth, the date of the meeting of 

 the Royal Astronomical Society, who passed a resolution 

 authorising their President to despatch a telegram of 

 congratulation to the President of the French Republic. An 

 important advance is thus made towards the attainment of the 

 ideal universal time, the minutes to be the same everywhere, 

 while the hours change in each zone of 15° of longitude. 



BOTANY. 



By Professor F. Cavers, D.Sc, F.L.S. 



SOME RECENT WORK ON GEOTROPISM.— Giltay 

 (Zeitschr. f. Botanik. 1910) has recently discussed and 

 criticised various methods of experimentation on the problems 

 of geotropism, and has called attention to a point which is often 

 overlooked by students of Vegetable Physiology, namely, that 

 we have never proved that gravitation is the only stimulus 

 in\-olved in the turning of the primary root towards the centre 

 of the earth. Long ago. Knight (1806) showed that as the 

 centrifugal force was increased on a centrifuge with a vertical 

 axis, the root and stem assumed a more nearly horizontal 



147 



