NATURE 



[July 4, 1895 



server, Mr. Roberts, have also been included in the new iata- 

 logue. {Astronomical foiinial. No. 347.) 



The Temi'ERatire of the SfN. — A new method of deter- 

 mining the temperature of the sun has lieen employed by H. 

 Ebert (Aslrophysical foumal, June). With the aid of data 

 supplied by Langley's investigalions, kubens deduced the law 

 that the wave-length of the maximum energy is inversely pro- 

 portional to the square root of the absolute temperature of the 

 radiating body. Experiments on the radiation of blackened 

 bodies between absolute temperatures 373" and ioSS° indicated 

 the relation 



AV''T=I23, 



T being the aljsolule temperature, and A being expressed in 

 microns (;i= '001 mm.). Langley has shown that the maximum 

 energy of the continuous background of the solar siiectrum is 

 very nearly at 0*6 /i, and assuming that the incandescent 

 particles in the sun which yield the continuous spectrum are 

 comparable to a black body as regards their total radiating 

 capacity, the application of the alwve formula gives a tempera- 

 ture of about 40,000' C. The ]xirls of the sun to which this 

 temperature applies are stated to belong to the interior regions, 

 below the photosphere. 



Dr. Ebert enters into a discussion of the electromagnetic 

 nature of the solar radiation, in order to justify the application of 

 the formula in the cise of the sun. This leads him incidentally 

 to suppose that the continuous background of the solar spectrum 

 is mainly due to hydrogen in a strongly compressed state. 



The Rotation ok Sati'RN. — In 1S93 Mr. Stanley Willi.ims 

 announced some highly interesting facts with reference to the 

 period of rotation of Saturn, as deduced from observations of 

 spots on different parts of the surface of the planet (Natire, 

 vol. I. p. 32). The observations were conlinue<l during the 

 opposition of 1894, and similar striking results have been arrived 

 at. {Monthly Notices, vol. Iv. p. 354). It was again found 

 that the sptjis indicated widely different rotation periods in the 

 same latitude, but in different longitudes, as shown in the 

 following table : — 



Range in longitude. 



30-130 



140-200 



,240-360 



f 0-80 



80-160 



[ 163-360 



The average rotation periods of the whole equatorial spot 

 zone during the four years of observation were as follows : — 



Dark spots 

 (I7°-37^N.: 



Bright spots 

 (6°S.-6^N.; 



Diff. 



43-6 



45 ■« 

 i6-6 



U.S. Weather Bureau. The law of emission for a white light 

 is that its visibility is proportional to the square root of its 

 candle-power, and the results of the experiments by the Com- 

 mittee closely follow the law, the departures being no greater 

 than the estimated errors of position of the vessel. The mean 

 of a large number of observations gave .is the distance at which 

 a while light of one candle-power became visible I -40 miles for 

 a dark clear night, and i 00 mile for a rainv one. Experiments 

 undertaken in -America, after the International Maritime Con- 

 gress in 1889, gave the following results in very clear weather : 

 .\ light of I candle-power was plainly visible at 1 nautical mile, 

 and one of 3 candle-power at 2 miles. .X 10 candle-power light 

 was visible with an ordinary binocular at 4 miles : one of 29 

 candles faintly at 5, and one of 33 candles visible without ditti- 

 cully at the same di.stance. On a second evening, exceptionally 

 clear, a white light of 32 candle-power could readily be distin- 

 guished at 3, one of 5-6 at 4, and one of 17-2 at 5 miles. The 

 Dutch governmental ex]>eriments, conducted at Amsterdam, 

 gave the following results : -V light of i candle-power was visible 

 at I nautical mile ; 3-5 at 2, and 16 at 5 miles. Experiments 

 with green lights gave O'So as the distance in miles at which a 

 green light of a single candle-power is just visible. The candle- 

 ix)wer required for a green light to be visible at 1,2, 3, and 4 

 nautical miles was 2, 15, 51, and 106, respectively. The Ameri- 

 can experiments before referred to give for green light : 3-2 

 candle-power fairly visible at I mile, and 28-5 clearly at 2 miles, 

 these results being, however, fron\ a limited number of experi- 

 ments. The German trials were much more numerous. The 

 extraordinarily rapid diminution of the visibility of the green light 

 with the distance, even in good observing wealher, and the still 

 more rapid decrease in rainy weather of a character which « ill 

 but slightly diminish the intensity of a white light, show that it 

 is of the utmost importance to select for the glass a shade of 

 colour which w ill interfere with the intensity of the light as little 

 as po.ssible. The sh.ade recommended is a clear blue-green. 

 Yellow-green and gra.ss-green sliouUl not be employed, as they 

 become indistinguishable from white at a very short distance. 

 Eor the red, a considerably wider range is allowable, but a 

 copi)ery-red is said to be the best. 



1891 10 14 21-8 



1892 13 38-2 



'893 '2 52-4 



1894 10 12 35-8 



The extreme difference of Im. 46s. observed since 1891 

 " means a very considerable increase in the velocity of motion of 

 the surface material, amounting to 66 miles per hour. In 

 other words, the great ecpiatorial atmospheric current of .Saturn 

 was flowing 66 miles an hour more quickly in 1894 than it was 

 in 1891." 



Taken as a whole, the observations indicate a more rapid 

 rotation of the planet in the equatorial regions than in the 

 northern zone of s|K)Is, and they appear to establish that there 

 arc great differences of velocity in different longitudes. 



To I'rof. Darwin, these results " suggest a rather wild con- 

 sideration " (fAjt-rr'rt/o/j, June). He considers it possible that 

 seriioiis of the planet |»irallcl to the equator may not be circular, 

 ill ' that it might be worth trying to detect systematic 



ill 'ween the various equatorial diameters by inicro- 



J/II-: vjsiiuLirY OF ships' ligi/ts. 



T T m.ay be remembered that in 1890, the Cicrman Marine t)bscr- 

 vatory lesleil some three thousand running lights in use on 

 Imard ships, and founil two-ihir'ls of them defective, l-'urlher 

 tests of the visibility of lights of known candlc-|X)wer were made 

 by the (jcrman Committee last year, and .some of the results 

 obtained arc noted in a leaflet just distributed to .seamen by the 



NO. 1340, VOL. S2] 



THE RELATIVE POWERS OF LARGE 



AND SMALL TELESCOPES LW SHOIVLXG 



PLANETARY DETAIL. 



T T is to be hoped that a definite understanding will soon be 

 * arrived at regar<ling the differences between large and small 

 telescopes in revealing delicate surface-markings on Mars, Jupiter, 

 and Saturn. The subject of relative efViciency «as discussed 

 about ten years ago, and some interesting evidence was evoked 

 as to the different forms and sizes of telescopes, but no settlement 

 of the question was possible in the face of the diversity of o]union 

 existing. The lime seems to have come when the subject may 

 be suitably referred lo, and the facts considered a]iarl from mere 

 prejudice or preference for any kind or size of instrument. 



The phenomenal results receiuly claimed for certain sni;iU 

 telescojK'S are almost of a character to shake even the faith of 

 those disposed to acknowledge their great utility -.n several 

 classes of objects, for our confidence caiuiol go beyond reason- 

 able limits. In individual cases a good thougli small instrument, 

 an acute well-trained eye, acting in combination with the best 

 atmospheric conditions, will yield surprising results ; but some 

 of those lately published border upon r<miance, and henceforth it 

 would seem that if all the data derived with .such means are to be 

 absolutely accepted, then large telescopes are grossly incapable 

 on certain important objects, and may as well be packed away 

 in the lumber rooms of our observatories. 



This is the more surprising when we consider the opinions 

 expressed during the discussion which previously took place on 

 the same subject. I'rof. C A. \'oung, who has charge of the 

 2V'nch refractor at Princeton, s:iitl : '* I can almost always see 

 with the 23-inch everything I see with the gj-inch under the 

 same atmospheric conditions, anfl see it better — if the seeing is bail 

 only a little better, if good immensely better." OiIkt observers 

 having the means of comparing large and small instruments, side 

 by side, furnished similar evidence, except in the case of .\l. 

 Wolf, of Paris, who said : " 1 have observed a great deal with 

 two instruments (both rellectors) of 15-7an<l 472 inchesaperture. 

 I have rarely found any advantage in using the larger one when 

 iheobjecl was sufficiently luminous." I'rof. ,\saph Hall, whose 



