I I J 



NATURE 



[November 23, 191 1 



was surprised to find the skin co firm and tough ; I almost 

 exp«;cted to spoil it in handling ; the delicate colours seem 

 as if they might be tender like the bloom of a grape. With 

 the smooth, but not sharp, edge of a knife I scraped the 

 surface somewhat vigorously without damaging the lovely 

 play ol bluf and green light. 



Two or three weeks elapsed before 1 could examine 

 dried pieces of the skin with a microscope. There did not 

 ;»p(>ear to l)e any orderly markings which could lead to 

 diffraction ; nor could I find a fl-iking tendency which 

 would suggest thin plates. The glaticing of the colours 

 from blue to green is lively, but I should think it is not 

 due to wave interference. Pigments can be greatly 

 modified by a change in the angle of incident light, accord- 

 ing to the qualities of lustre surfaces as described by 

 Holmholtz, and the polarisation principles worked out by 

 Jamin. These two considerations form the basis of an 

 important portion of the comprehensive discussion given 

 by Michelson. W. B. Crokt. 



Wini Iv-i'-i ('oll'-t,*-'. November 13. 



The Weather of 1911. 



With reference to Sir Edward Fry's letter in Nature of 

 November 16 in regard to the weather of 1911, without 

 presuming to reply on behalf of meteorologists to his 

 question as to whether any real cause can be assigned for 

 the brilliance and heat of its summer and autumn, may I 

 be allowed to suggest that the relatively high temperature 

 of the North Atlantic, during the period, presumably con- 

 duced to warmth and sunshine? Since April the surface 

 temperature of the North Atlantic has been, for the most 

 part, .tbove the average, and during the months of June to 

 September, inclusive, largely above. Not only through the 

 afi^ency of winds from seaward has air temperature over our 

 islands been raised by the abnormal warmth of the ocean, 

 but also, it seems probable, through the diminution of 

 cloudiness, and the corresponding increase of sunshine 

 attributable to the small difference, thus ensuing between 

 the temperature of the sea and that of the air above it. 



Moreover, during a bright summer, the heat which the 

 earth receives through solar radiation may be greatly in 

 excess of that which it parts with at night by terrestrial 

 ridiation. Campbell Hepworth. 



2 Amherst Road, Ealing, W., November 18. 



ROTATION PERIOD OF THE SC/iV.* 



C INCE the introduction of the Doppler-Fizeau prin- 

 •^ ciple one of its most interesting- and important 

 applications has been to the determination of the 

 velocity of the sun's rotation by the investigation of 

 shifts of the spectrum lines corresponding to various 

 points on the solar limb. Much of the pioneer work 

 in this direction was done by Vogel, Young, Langley, 

 Cornu, and Dunj^r, the results obtained by the latter 

 at Upsala from 1888 to 1903 serving as the standard 

 for future workers. Dundr's determinations referred 

 to points between the solar equator and 75° north and 

 south hcliographic latitude, the observations being 

 made at intervals of 15°. During 1903-6 Halm made 

 a series of observations at Edinburgh, employing the 

 same differential method as Dun^r. 



All these observations having been made by visual 

 methods, it was considered of importance that the 

 photographic method should be tried, and this was 

 commenced on the completion of the Snow telescope 

 on Mount Wilson in the spring of 1906. In June, 

 iqo7, the tower telescope, with vertical spectrograph 

 of 30 feet focal length, was completed, and thence- 

 forward the observations for rotation were made with 

 it, the spectra being of such greatlv increased resolv- 

 ing power that the investigation ' was considerablv 

 facilitated. 



»/ 'l^r 1"^^'"??,'"'°" ^'" ''^^ Rotation Pf nod of the Sun by Speciro<;copic 

 Methods By W S. Adams and I. S. Lasbv. Washington dTc . Papers 

 of the Mount Wilson Solar Observatory. Vol. i., part i. (The Cameeie 

 Institution, 191 1.) " 



Five series of observations have been made 

 dealing with lines selected from the spectrum < 

 general reversing layer, two with the Ho lii 

 hydrogen, and the fifth with the blue line of c;: 

 at X 4227. As the lines ot hydrogen and calciun 

 results differing widely from those obtained f( 

 reversing layer, the various sections are ti 

 separately in the discussion. 



The 1906-7 series of observations were made ii 



junction with the Snow telescope, having a spec 



scale of I mm. =o"7iA, on a list of lines seleci- 



being representative of the different strata inv^ 



Those made in 1908, with the tower telescop< 



spectrograph of 30-feet focal length, are with a 



trum scale of i mm.=o'56A. For the reversing 



photographs twenty-two standard solar lines 



i selected for observation, representing the element 



i Cr, Zr, Fe, Mn, Ni, Ti, and the compound CN 



' In the general discussion it is shown that the 



I series of observations are in substantial agn 



I for latitudes between 0° and 50°. .-Xbove 50' 



NO. 2195, VOL. 88] 



he m 



7\ 







Fig. I. — Curves showing variation of angular velocity with latiiu 

 sun-spots, faculx, calcium flocculi, reversing layer, A 4327, and . 

 To facilitate comparison the curves are all reduced to the co ~ 

 origin of 1 4" 50°. Differences in the amount of equatorial ao 

 tion are indicated by lack of parallelism in the curves. 



1908 observations give slightly smaller valuesj 

 greatest difference being about 0*039 km. at joi 

 is surmised that these small differences may be 

 due to small systematic errors in the earlier 

 and partly to actual proper motions in the va 

 constituting the sun's reversing layer. 



The general results do not indicate any de 

 variation of the sun's rate of rotation, except it 

 long period. This conclusion is emphasised 

 close agreement of the Mount Wilson results i 

 those given by Dun^r. 



One of the most notable results brought out 

 by these observations is the fact that the lit 

 different elements give different values of the ro| 

 velocity. Thus lines of lanthanum and cyanoget 

 low velocities, while certain lines of manganes 

 iron give comparatively high velocities. A tal 

 given showing the residuals on forming the differ 

 in the values of the angular velocity for each line] 

 that of the mean of all the lines. The syste 

 variation is at once rendered evident by the 

 preponderance of positive or negative residuals, 

 results corresponding to the different elements ar 



