scfENCE-aoss/r. 



33 



( II AIIKUS 1-UU VOL'Ni; ASTKOXOMKKS. 

 Hv Fka.nk C. Dusxt-rT. 



TiiK. sr\. 



Till, yrc.a iiiki nf mir systi'iii, whicli slicds Imili 

 lijjlil and Ileal llinuiyhdiit ils family iif larjjc ami siiiall 

 planetary spheres, claims very special altenlion. 



It is so great in size that if 1,300,000 globes the 

 siae of our earth were niade into one worlil, it wouUl 

 be no more in hulk than the sun. The density of the 

 sun is however much less, for 330,000 globes of the 

 same mass as the earth, would just about balance the 

 scale with the sun, so that it would take nearly four 

 cubic feet of the sun's body to eijiial one cubic fool of 

 the earth's. 



The marvellous brilliance of the disc renders the 

 utmost care necessary in taking observations with the 

 telescope, the objective of which really acts as a 

 burning glass. So great is the amount of heat that 

 I have had an ordinary dark glass blistered, and spoiled, 

 through I he heat Condensed byan object-glass only 25in. 

 aperture. The sun's altitude did not at the time 

 exceed 20 , and was therefore shorn' of much of its 

 heat. Dawes considered that 2in. aperture, of 3oin. 

 focus, was as large as could lie safely employed with 

 dark glasses. These small glasses will show much 

 detail, still every increase of aperture enlarges the 

 amount. Some observers have sought to put a dark 

 glass a distance up the tube, so that the rays may pass 

 through it befi^-e reaching the focal point. This plan 

 is however not good, because of the difficulty of 

 obtaining these glasses sufficiently perfect to be thus 

 used. The solar reflector alluded to in the last chapter 

 is much better. It is, however, necessary to remember 

 that a single reflection reverses the image in one 

 direction but not in the other. Another capital device 

 for reducing the light is a Uarlow lens, having a thin 

 film of silver deposited upon it, which reflects much of 

 the light and heat back up the tube. SirW.IIerschelused 

 to reduce the light by permitting it to pass through a 

 vessel containing filtered ink and water. Cooper, of 

 Markree Castle, used a "druii. " filled with alum 

 water to gel rid of the heal when employing his I3"3in. 

 achromatic, whilst dark glasses reduced the brilliance. 

 Dawes' method of using minule apertures in the 

 diaphragm is not to be recommended. .Many 

 observers, including I'rofessor .S. I'. Langley, the late 

 Miss li. Hrown, Captain Noble, and ihe I\ev.' 1'. 

 llowletl have frc(|uently used the telescope as a 

 camera lens, and focussed the image upon a disc of 

 paper or plaster of Paris. This method removes all 

 danger during the geneial examination of the surface, 

 but direct observation is better for the more minute 

 structure. The ilUi>tr.ition given shows the method 

 of filling the white screen on the telescope. There 

 must, however, be some method employed to prevent 

 the sun from shining directly on the paper. A large 

 disc of cardbiard with a central hole just large enough 

 to slip on tightly in the place of the object glass capi 

 is of use, but still better is a light "camera" of 

 aluminium 01 c.irdhoard, filling round the evepiece 

 and disc. This should be pierce<l with a liole through 



which obsvrvntiun can be made. Many olhcr devices 

 have iK'cn tried, but for the praclio^il |iur|xises of ihe 

 amateur these three, the solai reflector, the silvered 

 Harlow lens, and the projected image, are the only 

 ones available. The besi linl for dark glas.scs is 

 either green, or a neutral tint ; but never red or 

 yellow. Where cx()ense is no object, and a telescope 

 can be set a|)iiri for .solar work, .Mes.srs. Home and 

 Thornthwaite have fitted up a Oin. Newtonian 

 reflector with unsilvered glass mirrors, the bottom 

 surface of the large speculum being curved so that 

 there are no troublesome reflections. This method, 

 whilst it reduces ihe light ami heal very greatly, still 

 makes n large aperture available. Whatever 

 arrangement is adopted, do not allempl to look 

 directly at the sun through a telcscoix; of any size, 

 with only dark glasses. This indiscretion cost Sir W. 

 Herschel the sight of one eye. Only recently I saw 

 a dark glass which had simply been "'burned up" 

 with the solar rays, condensed by the 6jin. Sheep- 

 shanks telescope at Greenwich observatory. 



Solar <ibservalion, to be thorough means close work, 

 because of the great rapidity with which changes 

 take place. If it be possible lo observe several times 

 a day, the astronomer's labours will be rewarded. 



An equatorial mounting, and cross lines in the eye- 

 piece, render observation v<;ry much more easy, 

 because the exact position of a spot on the sun's disc 

 can be more readily determined ; a point of very 

 considerable importance. 



The general surface of the sun is not, as it may 

 seem at a first glance, of even brilliance, but is more 

 like the face of a lit|uid covered by closely packed 

 fluculent-looking patches which it would be almost 

 impossible lo lailhfully reproduce in an engraving. 

 .\niongst these "flocks" there occasionally appear 

 duller patches, whilst singular patterns sometimes 

 seem to be formed amongst them, as w.is long since 

 pointed out by Sir William Huggins. Nasniyth 

 thought the appearance was brought about by the 

 interlacing of multitudes of elongated bodies, whi'ch he 

 considered were best described as "willow-leaves," bul 

 later observations have not verified this proposition. 



The solar atmosphere, if it may be so described, 

 makes its presence very visible by reducing the 

 brilliance of the edges of the disc, as compared with 

 the centre. This eflecl is readily seen on many 

 photographs, and also is very visible when the image 

 is projected on the screen above described. 



l£ven before the invention of the telescope, from 

 time lo time when the sun was \ety low down, it was 

 noticed that occasionally dark spots were present on 

 ihe disc. Records of such spots are to be found as 

 early as among the writings of the ancient Chinese. 

 These were of course observed without optical aid. 

 The dark spots may often be seen with the naked eye 

 when the sun is low, and has lost his brilliance. If 

 the spots are frequently observed it will soon be 

 noticed that they are never seen near the poles nor on 

 the ei|Uator. It will also be noticed that the spots 

 often appear round the eastern limb, and then from 

 day to dsy lo get further from the eastern and nearer 

 the western limb. The complete transit across the 

 disc occupies about twelve (la\s. It will be noted 

 that the transit does not always occupy the same time, 

 in other words all parts of the disc do not revolve in 

 the same lime, so demonstrating that the visible sur- 

 face of the sun has not a solid surface as has our ow n 

 world. 



( To he continued. ) 



