<'eb. 1, ItsSC] 



* RnowLedge 



115 



as this such a. record can be complete) for the must 

 conscientious student, it has all the interest of a grand 

 romance for the reader who not being a student of geology 

 takes thoughtful interest (as every cultured person must) 

 in the results of scientific research. 



We shall return, and more thr,n cnce, to the subjects 

 dealt -with in the various divisions of this -work. In the 

 meantime Tve commend it -syarmly to those among our 

 readers who have not 3-et seen it. Those who have will 

 need no recommendation. The new edition hss been 

 much enlarged, and carefully brought up to date in 

 regard to geological and palivontographical matters. The 

 book might well bear (he title which heads this paper. 

 When shall we have a " Story of the Heavens " as well 

 done ? 



OPTICAL RECREATIONS. 



By a Fellow op the Rotal Asteonomical Society. 



colour and light. 



I W, before proceeding any further, we must 

 stop to caution the student that the theory 

 of three primary colours expounded in our 

 last chapter is, and can be, oulj> what the 

 metaphysicians call "subjectively" true: 

 in other words, it is only so as regards 

 our oicii stnmlions. A verj' little thought 

 will suffice to show that, externally to ourselves, 

 there can be no such thing as three elementary 

 kinds of coloured light ; for, from what has pre- 

 ceded, it must be obvious that many hundreds of 

 difierent tints are existent in the spectrum — each with 

 its appropriate wave-length—to say nothing of the 

 waves at both ends of it, which fail to affect our 

 vision at all. Once more, then, light consists of nothing 

 but a scries of undulations or vibration.s, the length r.nd 

 freijuency of which, when they impinge on the retina, 

 affect us with the various sensations of colour. We 

 know that when the rays of the .'un fall ujjon the nerves 

 of sight we are affected with the sensation of light ; 

 whereas the identically same rays, exciting our nerves of 

 touch, produce in us the sensation of heat. And so in 

 Young's theory, all that is postulated is that the dif- 

 ference of our sensations of colour has its origin in the 

 kinds of nervous fibres which are excited. When all three 

 kinds are equally stimulated we see what we call white 

 light. Colour, in the sense in which we know it, can 

 have no existence whatever outside of ourselves ; though 

 the actual waves in the Kther of every conceivable lengfh, 

 of course, both can and do exist. "With this qualifica- 

 tion, then, we will resume the thread of our description. 

 We said on page 96 that red and greenish blue, orange 

 and a greenish blue, bright blue and bright yellow, and 

 greenish yellow and violet light, when mixed in the 

 indicated pairs, all four give white light as a result. It 

 will, then, only seem natural that if, by any means, we 

 subtract one of these colours from white light, the other, 

 its so-called " complementary " colour, will remain ; and 

 this we shall find to be the result of direct experiment. 

 Now, an accurate knowledge of what these complementary 

 colours severally are is of the greatest practical imiiort- 

 ance to the artist, since, as we shall find, they afford the 

 very strongest contrasts that it is possible to obtain, when 

 placed in juxtaposition. One of the most simple methods 

 of obtaining truly complementary colours is by the aid of 

 the polariscope and thin films of selenite. Another way, 

 by a system of trial and error, is to employ the double- 

 image prism shown in Fig. 8 (p. 96), at P. Placing a 



parallelogram of paper coloured of the tint whose com- 

 plement we wish to obtain on a dead-black g-ronnd, we 

 paint a similar piece of what we suppose" should be 

 the complementary colour ; and, laying it by the side of 

 our first piece, cause their duplicated images to overlap 

 by regarding them through our prism. If the resulting 

 image be not -white (or rather, using coloured paper 

 pure grey), we see what colour predominates, and 

 suppress this if it leaves our first tint outstanding, and 

 vice-versa, until we hit upon a shade which exactly 

 neutralises our first one. Or, preferably, we use Maxwell's 

 discs (Fig. 6, p. 9.5), because from their construction they 

 enable us to add more than one colour to our compensating 

 sector, and so ultimately to obtain a mixture thr.t shal?, 

 with our original one, accurately make up white— or, as 

 we have said above, pure grey — light. Suppose, for 

 examp.e, that we wish to find the complement of the 

 crimson of the spectrum, we shall discover that neither 

 blue nor bright green will serve our purpose, but if we 

 add a green sector of the proper size to our blue one, we 

 shall find that the two in combination will neutralise our 

 crimson, and produce white light (or its equivalent, in 

 our apparatus, pure grey). In the same way a purple 

 truly complementary to green may be arrived at, and so 

 on. Or, finally, there is the rough-and-ready method 

 hinted at at the conclusion of our last paper — that of 

 tiring the retinal nerves out by gazing intently at any 

 given colour, and then turning the eye towards a while 

 surface; from which the temporarily colour-blind retina 

 will select the colour to which it is fresh. Almost every 

 one, we suppose, has tried this experiment in the form 

 of staring at a red wafer on a white sheet of note- 

 paper, and suddenly removing the wafer, when the 

 spot it occupied on the paper will appear as a 

 green one. The explanation of this, after what 

 has preceded, must be obvious. The spot on the ret ina 

 on which the image of the wafer has fallen has become 

 tired out as far as red light is concerned ; and when tbo 

 exciting red image is suddenly withdrawn and the spot 

 it cccupied is filled with white light, the fatigued nerve 

 fdlsto detect the red element in the white light, but 

 picks up the green one readily enough. In a similar 

 fashion if we steadily regard a well-lighted brightly- 

 coloured picture for some time and then turn the eyes 

 rcipidly towards the ceiling we shall see its "after- 

 image " in complementary colours. A striking way of 

 showing the existence of retinal fatigue is to put a sheet 

 of note-paper on a dark background and cover half of it 

 with a piece of dead-black card, fixing the eye steadily 

 upon a point about the middle of the boundary of light 

 and darkness. The paper should be well lighted. After 

 gazing at this arrangement for a minute or two, if the 

 black card be suddenly removed, that half of the paper 

 which is previously covered will seem very notably 

 brighter than the other one which has been continuously 

 under observation. 



We have now seen how the sensation of white light is 

 produced by the excitation of all three of the retinal 

 elements whose vibrations produce in cur brain the sen- 

 sations of red, green, and bluish-violet, or violet-blue ; 

 ami further, how a very considerable range of the mul- 

 titudinous colours observed in nature is producible by 

 the mixture of these primary colours in various propor- 

 tions. Considerable however as this range of colours 

 is, it is very far indeed from exhausting all the tints 

 and hues we behold around us, which, at first sight, it 

 seemingly should do, were the colours we have specified 

 really ultimate ones. But, so far, we have proceeded on 

 the assumption that their action on the eye never varies, 



