November, 1902.] 



KNOWLEDGE 



245 



with the smell of rotten eggs, gave deuse condensation 

 after being exposed to sunshine. Clilorine was one of the 

 most interesting of the gases tested by Dr. Aitken, as it 

 caused condensation to take place without supersatu- 

 ration. 



In his investigations. Dr. Aitkeu considered how long 

 these nuclei, due to the action of sunshine in certain gases, 

 would remain active for cloudy condensation. But he is 

 not yet prepared with very satisfactory results. Some are 

 very short-lived — fifteen minutes to half-an-hour being 

 sufficient for the diluted hydrogen peroxide in the flask 

 to lose its power of cloudy condensation ; while the nuclei 

 from sulphurous acid remained active for a considerable 

 time. 



He IS now convinced that it is ])ossible for cloudy 

 condensation to take place in certain circumstances iu the 

 absence of dust. This seems panwlo.xical when we 

 remenilx>r Dr. Aitken's beautiful experiments twenty years 

 ago ; but in ordinary circumstances dust is needed for 

 cloud formation. However, supposing there is any part 

 of the upper air free from dust, it is now found possible, 

 if any of the g-ases experimented on be present, for the sun 

 to convert them into nuclei of condensation, and permit 

 of clouds forming in dustless air, miles above our atmos- 

 phere. 



In the lower atmosphere there are always plenty of dust- 

 l)articles to form cloudy condensation, whether the sun 

 shines or not. But in the higher atmosphere, and iu the 

 region above our atmosphere, clouds can be formed by the 

 action of the sun's rays on certain gases. This is a great 

 boon to us on the earth, for it assures us of clouds to 

 defend us from the sun's extra powerful rays, even when 

 our atmosphere is fairly clear. Dr. Aitken's communica- 

 tion is of some meteorological importance ; but he is 

 cautious, and we wait for his more matured conclusions. 



ASTRONOMY WITHOUT A TELESCOPE. 



By E. Walteb Maunder, f.r.a.s. 



XIX.— THE COLOUES OF STARS. 



In concluding my review of departments of observation 

 open to the "astronomer without a telescope," I wish to 

 glance at one which offers him some small opportunity, 

 although in general jt must be considered one for the 

 possessors of telescopes and those even of considerable 

 size. 



The wide difference which there is between star and star 

 as to brightness is apparent on the very first glance 

 towards the heavens ; it requires a more careful scrutiny 

 to realize that they differ also in their colour and in the 

 character of their shining. The ancients carried their 

 discriminaiiou of the difference in the brightnesses of stars 

 so far as to recognise six magnitudes, and added the further 

 refiu<!meiit that they noted many stars as being somewhat 

 brighter or somewhat fainter, as the case might be, than the 

 average star of their magnitude, but when it came to the 

 question of colour they hardly noted any differences at all. 

 The stars in general were described as yellow, six only being 

 recorded as " fiery." Of these six we should class five as 

 being distinctly orange or I'ed — Autares, Betelgeuse, 

 Aldebaran, Arctunis and Pollux. The sixth, Sirius, is to 

 us an intensely white star, and there have been many 

 discussions as to whether it has changed its colour in the 

 last 2000 years, or whether the description given of it — 

 " fiery red " — is due to some mistake in the record, or 

 whether the excessire scintillation of the star may account 

 for it. For, as we see it now when near the horizon, a 

 momentary flash of vivid red flames out from time to 



time, due to the irregular dispersion of its light in passing 

 through the tremulous atmosphere. It is from this that 

 Tennyson, most exact of all the poets in his scientific 

 references, calls Sirius " fiery " in the well-known passage 

 from the " Princess " : — 



" The fiery Sirius alters hue 

 And bickers into red and emerald." 



A careful comparison of star with star will soon show 

 that this classification is far from exhausting the differences 

 of tint which may be recognised amongst the stars visible 

 to the naked eye. Indeed the Star Colour Section of the 

 British Astronomical Association, under the able and 

 energetic direction of Mr. W. S. Pranks, f.e.a.s., some 

 few years ago drew up a catalogue of all the stars of the 

 fifth magnitude and brighter, and recognised no fewer tliau 

 twenty different tints or shades. The immense majority 

 were indeed white, or white with a more or less evident 

 tinge of yellow, up to a fairly full yellow, and the observa- 

 tions were made tor tlie most ]3art by means of refractors 

 of three to four inches in aperture, or of reflectors of six 

 to eight inches. The observer, therefore, who has no 

 telescope, or who at best possesses but a good field-glass, 

 would neither be able to deal with stars so faint as the 

 fifth magnitude, nor to detect as many differences of 

 colour. 



Nevertheless, with a good field-glass some three hundred 

 stars would be within his reach, and with the naked eye 

 alone quite one hundred, the colour of which he might 

 successfully estimate, and in all probability with patience 

 and experience he would succeed in grouping these iuta 

 fully a dozen different categories. The work would soon 

 be felt to be an attractive one, for delicacy of discrimina- 

 tion would be sure to come with practice ; and the sense 

 of the power to discriminate with Cjuickuess and certainty 

 between two stars which at first glance showed no difference 

 would certainly bring pleasure. 



In the work of estimating the colours of stars, there are 

 two different points to keep in mind. The one is, what 

 point lu the spectrum should be taken as best representing 

 the dominant tint of the star. The other, what is the 

 intensity of that tint, for it must be remembered that the 

 stars give continuous spectra, that is to say, the ground- 

 work of their spectra is essentially continuous. Koughly 

 speaking they all give light of all wave-lengths ; iu other 

 words, they shine essentially by white light. We have no 

 stars with spectra limited to one particular region. Even 

 in the banded spectra none of the seven colours that we 

 ordinarily recognise arc entirely wanting, and even it' we 

 went further, as we easily might, and divided the spectrum 

 not into seven only, but into twelve or more different 

 colours the same statement might hold good. 



We must therefore regard the stars as shining 

 essentially by white light. But the various tiuts which 

 together go to make up a perfect white, are not always 

 found in their exact proportion. We may regard, there- 

 fore, a coloured star as a star shining by white light plus 

 a certain projjortion of light of one or more specific 

 colours. 



Assuming that this is so, that the light of any star is 

 partly white and partly coloured, we may divide the stars 

 into classes, depending entirely upon the depth of tint 

 which they show, and not upon its colour. A five-fold 

 division suggests itself, something to the following 

 effect: — (1) pure white, {'2.) tinted, {o) coloured, (4j fully 

 coloured, (5) deeply coloured. Amongst the stars visible 

 to the naked eye the full and deep colours are rare ; it is 

 especially in the field of doable star astronomy that we 

 get the deeper and ri<her tints. 



After the question of the depth of tiat which the stars 

 show, comes the question of the colour of that tint. For 



