246 



KNOWLEDGE, 



[N0\'EMBEB, 1902. 



naked-eye stjii's, the more refrangible colours do not come 

 into consideration. The ranf,'e is from orange-red up to 

 yellowish green, or possibly in a single instance — that of 

 Beta Librw — to green. Alpha Lyrw, and possibly one or 

 two other stars, have a distinct bluish tinge, but in general 

 the stars not passed as white may be very well scheduled 

 under one of the live following hi'ads:^(l) reddish- 

 orange, (2) orange, (3) orange-yellow, (4) yellow, (5) 

 yellowish green. 



In working upon star colours vritli the field-glass or 

 naked eye it is impossible to use any artificial standard of 

 colour, but the wide field of view, and the ease and 

 rapidity with which the attention can be turned from one 

 part of the heavens to the other, will much more than make 

 up for this deficiency. The stars must be compared one 

 with another, and the estimations of colour must be purely 

 relative, and the method will be found much the most 

 accurate possible. 



The most satisfactory programme for an evening's work 

 would probably be to take a number of stars not differing 

 too widely in magnitude, and without regard to the exact 

 name to be given to the depth of tint of each, to arrange 

 them simply in order of colour intensity, beginning with 

 the star of purest whiteness and going downwards to the 

 one that showed the fullest tint. It would probably be 

 found well for the sake of simplicity to keep this work of 

 the arrangement in oi'der of the depth of tint entirely 

 separate from the work of arrangement in order of colour. 

 For several nights a large number of stars should be taken 

 and carefully compared, each one with the others, until a 

 satisfactory progression has been arranged for the whole 

 number ; the depth of tint being the criterion. 'J'hen, 

 when this work has been carried out successfully, a similar 

 comparison should be instituted as to the colour of the 

 stars, ranging them in order from the reddest to those 

 which show the nearest approach to green ; and in this 

 second comparison it would probably be found useful to 

 confine the work at first only to the stars of deep tint, 

 then to those of full colour, and so on, until last of all 

 those arc classified according to the spectral order of their 

 colours which are only slightly tinted. Thus, if a hundred 

 stars have been selected as showing some more or less 

 appreciable tint, and have been arranged in order of the 

 depth of tint, number 100 being the star of deepest tint, 

 then numbers 61 to 100 might be the subject of the first 

 arrangement in spectrum order, numbers 41 to 80 of the 

 second, and so on. 



The great advantage of making the colour estimations 

 purely comparative would be that in the case of auv 

 variation of colour in any star its detection would be 

 easier, and much more free from ambiguity than by any 

 other method. Thus Klein and Weber state that Alplia 

 Ursai Majoris passes through a series of colour changes in 

 a period of 33 days. Several telescopic stars are supposed 

 in like manner to have changed their colour. The case of 

 Sirius has already been referred to, but whatever it may 

 have been in classical times, it certainly has shown itself 

 unvarying white in our own day. Algol, also a white star 

 to us, is described by Al Sufi as " red." It is therefore 

 not impossible that the systematic study of star colours, 

 even though it be undertaken by the naked eye alone, or 

 at best with the assistance of a field-glass, may yet succeed 

 in discovering an instance of colour change. But should 

 it fail to do so, it must not be forgotten that the clear and 

 unmistakable evidence, which a thorough and systematic 

 record of the relative colours of stars would supply, that 

 no colour change had occurred, would be just as valuable, 

 just as important. 



Closely connected with the colour of stars is their 

 mode of shining; in other words, their scintillation. 



The blue stars and white stars, like Vega and Sirius, 

 " twinkle" the most rapidly ; the orange stars shining the 

 most steadily. The planets ordinarily do not scintillate. 

 This scintillation is, of course, due to tremors in our own 

 atmosphere, since the stars are to us absolutely mathe- 

 matical points to whi(^h not even the most ])owerful 

 ti;lcscope has yet succeeded in giving any ap()reciable 

 diameter. It follows that when the air through which 

 the star's rays pass to our eye is in great agitation, many 

 rays may reach us one moment and few the next, and the 

 star may seem to flicker like a candle in the wind. The 

 ])lanets do not scintillate because they show to us real disks, 

 even though these are not large enough to be perceived as 

 sucii by the naked eye. The one exception, the planet 

 Mercury, is an exception partly because it is always near 

 the horizon when seeu with the naked eye, and partly 

 because its diameter is very small. It owes its Greek 

 epithet <rri\pu>v, " glittering " or " flashing," to this 

 peculiarity amongst the older planets. 



The measurement of the amount, or, rather, rapidity of 

 scintillation, lies beyond the power of the " astronomer 

 without a telescope," but even to the naked eye there is a 

 marked difference in this respect between star and star, 

 and the observer will not be able to overlook its evident 

 connection with the star's colour. 



The review of the various departments of observation, 

 available to the unassisted sight, has thus brought us to 

 studies, as in the case of variable stars and star colours, 

 where, though there is a certain amount of work to be 

 done without optical aid, yet the observer's field is much 

 increased by the possession of a good opera-glass or field- 

 glass. 



We have come as it were to the threshold of that 

 noble structure which has been built up by means of the 

 telescope and spectroscope ; but to that building there are 

 many guides, and to explore it lies beyond my present 

 task. The purpose which I have set before myself has 

 been a much humbler one, but if I have succeeded in 

 arousing interest in those astronomical phenomena which 

 require no " optic glass " for their display, I shall be well 

 rewarded. 



And I think that he who seriously undertakes some 

 department of astronomy without a telescope, will likewise 

 not fail of his reward. The growth in the power of per- 

 ception which careful practice in observation brings is a 

 real gain. Real too is the gain in habits of system and 

 method. For to perceive is but a part of the astronomer's 

 work ; he must learn to record what he has perceived, and 

 must form the habit of recording at once and recording in 

 order. And this habit, as well as the gain in perception, 

 means increase of power, and power gives pleasure. 

 Pleasure there is too in gaining as it were from direct 

 converse with nature, fresh insight into her mysteries ; 

 pleasure, if our knowledge is really increased ; pleasure, 

 too, even if the problems prove too involved for us and 

 our only progress be towards a truer ajipreciation of their 

 difficulty. 



The fields of work which we have passed in review have 

 been both many and varied. They have extended from 

 phenomena the most slight and transient — the lighting of 

 a sunset cloud, the momentary flash of a meteor — to the 

 greatest and most enduring that the universe can show — 

 the fabric of the Galaxy and its interweaving with the 

 stars. And there is above all in this direct study of the 

 heavens, out in the open, beneath the deep unsoimded 

 sky, a charm and an awe, not to be realised otherwise. It 

 is "nature at her vastest that we apjjroach ; we look up to 

 her in her most exalted form. We see unrolled before us 

 tlie volume which the finger of God has written ; we stand 

 in the dwelling-place of the Most High. 



