Jan. 1, 1886.] 



♦ KNOAVLEDGE 



97 



than you put into it. Pretty evidently, then, the total 

 luminosity of the spectral coloured disc produced by 

 rotation will be the moan of the luminosities of its two 

 painted components. Suppose now that we diminish the 

 intensity of our green light, then -will the yellow pro- 

 duced by its combination with the red become orange, 

 passing as such diminution progresses through red- 

 oi'iinge and orange-red to pure red. It will be at 

 once inferred that if we, on the other hand, diminish the 

 intensity of the red component, the resulting colours of 

 the combination will pass through greenish yellow, 

 yellowish green, and so to pure green. Similarly, by 

 mixing green and violet light, we shall produce the whole 

 range of blues, and so on. We have said above that, 

 while the theoretical three sets of nerves are to a very 

 great degree respectively sensitive only to the three 

 primary colours — red, green, and violet — yet that each is 

 also affected slightly by the two remaining colours not 

 specially pertaining to it. Hence, under ordinary cir- 

 cumstances, a very small quantity of white light must 

 be mixed with every colour we perceive, diminishing to 

 that extent its intensity' or saturation. And that this is 

 the case even with the colours of the spectrum a simple 

 experiment, devised and described by Helmjioltz will 

 suffice to show. We can tire a nerve just as we can tire 

 a muscle. If we go out into the bright sunshine, and 

 then enter a darkened room, for some time we can see 

 nothing at all. Our retinal nerves are so fatigued with the 

 glare of the sun as to be absolutely insensible to the faint 

 light reflected from objects in our dimly-lighted chamber. 

 By and bye, when the eye becomes, as we say, accus- 

 tomed to the darkness, we can see perfectly, and even 

 read, where on our first entry nothing whatever was 

 visible. On a cognjite principle, if we stare at a bril- 

 liantly-lighted church window for a minute or so, and 

 then turn the eyes towards the grey wall, we shall see 

 an image of the window of the identicrA size and form 

 of the original, but with white for black and bhick for 

 white, like the negative of the photographer. The gkre 

 of the light transmitted through the glass has tired out 

 the retina, and those parts of it upon which that glare 

 has fallen are less sensitive to the light reflected from 

 the grey wall than its other portions, hence there 

 appears the dark image of the window-panes upon the 

 uniform surface. Thus much in preliminary explanR,tion 

 of the experiment of Helmholtz, who gazed upon bright 

 bluish-green until he quite tired out his retina as far as 

 that colour was concerned, making himself, in fact, prac- 

 tically green-blind and violet-blind. Regarding now the 

 red of the spectrum, the after-image of the bluish-green 

 appeared of such a burning and effulgent red that the 

 adjacent red looked actually whitish in comparison ! 

 . We shall have something more to say of coloured 

 "ghosts" in a future paper in connection with the 

 subject of complementary colours. We shall see too how 

 indefinitely our range of colours producible by mixtures 

 of the three primary ones, is extended by mere changes 

 in luminosity. For the present, what we have here said 

 must suffice. 



Royal Victoria Hall, Waterloo Bridge-road, S.E. — A 

 grand Eastern spectacle, entitled " King He's Abore, the Plague of 

 His Dear-mall," is being prepared for the Christmas entertainment 

 at the above hall. It is replete with splendid scenery, including the 

 Enchanted Valley of Palms and Precious Gems, the Abode of Evil 

 Spirits, the Grand Palace of King He's Abore. Beautiful dresses, 

 electrical and sensational effects will be amongst tlie attractions of 

 the entertainment, which will be produced under the personal 

 superintendence of Jlr. Frank Wright, assisted by a company 

 selected from the principal artistes of London and the provinces. 



THE STORY OF THE HEAVENS,* 



HIS is a most disappointing work. We 

 opened it rejoicing in the thought that 

 now at last we had a volume which we 

 could nime in answer to the multitudinous 

 inquiries of young students for a really 

 trustworthy and at the same time in- 

 teresting and instructive treatise on 

 astronomy — yet with some misgivings that as in other 

 cases where survej-ing astronomers had attempted the 

 work of astronomical teaching a rather dry and un- 

 inviting book might have been produced. But our 

 disappointment has been more complete than this natural 

 fear suggested as possible. The book before us has little 

 interest in the way of descriptive astronomy, its supply 

 of solid food is of the most indigestible sort, and the 

 theoretic and speculative matter is not only very ques- 

 tionable but relates to subjects of very little interest. 



One or two examples of Dr. Ball's method in descrip- 

 tion and in explanation will be fairer than mere verbal 

 criticism. As to the theoretical matter we note that 

 beyond Tschermak's theory of meteoritic origin (un- 

 soundly limited to ejection from the earth), and the 

 theory of the moon's origin from eartli-waves (which the 

 time-element negatives, as here advanced) there is little 

 to attract attention. 



Here, then, is the description of those most interesting 

 of all celestial objects the rings of Saturn : — 



" The various features of the rings are well shown in 

 the beautiful drawing of Trouvelot. We here see the 

 inner and the outer ring, and the line of division between 

 them. We see in the outer ring the faint traces of the 

 line by which it is divided, r.nd inside the inner ring we 

 have a view of the curious and semi-transparent crape 

 ring. The black shadow of the planet is cast upon the 

 ring, thus piroving that the ring, no less than the body 

 of the planet, shines only in virtue of the sunlight which 

 falls upon it. This shadow presents some anomalous 

 features, and its curious irregularity may be, to some 

 extent, an optical illusion. The drawing contains no 

 trace of those other and finer lines which are more or 

 less problematical, but it is a faithful representation of 

 the planet, under good seeing conditions, and viewed in r. 

 telescope of considerable power. There can be no doubt 

 that any attempt to depict the rings of Saturn can only 

 represent the salient features of that marvellous system. 

 We are situated at such a great distance that all objects 

 not of colossnl dimensions are invisible. We have, indeed, 

 only an outline, which makes us wish to be able to fill in 

 the details. We long, for instance, to see the actufd 

 texture of the rings, and to learn of what materials they 

 are made; we wish to comprehend the strange and filmy 

 crape ring, so unlike any other object known to us in the 

 heavens. " There is no doubt that much may even yet be 

 learned under all the disadvantageous circumstances of 

 our position ; there is still room for the labour of whole 

 generations of astronomers provided with splendid in- 

 struments. We want accurate drawings of Saturn under 

 every conceivable ;;spect in which it may be presented. 

 We want incessantly repeated measurements of the most 

 fastidious accuracy. These measures are to tell us the 

 sizes and the shapes of the rings ; they are to measure 

 with fidelity the position of the dark lines and the 

 boundaries of the rings. These measures are to be pro- 

 tracted for generations and for centaries ; then and then 

 only can terrestrial astronomers learn whether this 



* " The Story of the Heavens," by Rob. S. Ball, Astronomer- 

 Royal of Ireland. Cassell & Co., London, &0. 



