RECREATIVE SCIENCE. 



97 



etako in thoir city that they burnt it, rather than capitulate, when 



<i by too Noritei! i was in ilm t.-ntli m. 



<>f a trade with tin- Icliiii.'-Unds, Saxony, Denmark, Nor- 



voden, and It had oommoroial relations with 



Cologne, and was one of the first ports to engage in the wine 



trade. Besides the weaving of silk, linen, and wool, the citi- 



sena were employed in dyeing and bleaching, and in the making 



ion ware. Agriculture was profitably pursued in the out- 



Amsterdam, on the Amatol, was in tho year 1300 little more 

 than a hamlot, sheltering a few poor fishermen who obtained 

 thoir scanty living from the Zuydcr Zee. The neighbourhood 

 was desolate marsh-land. Upon this unpromising site the city 

 arose, and became in time the capital of tho commercial world, 

 roatness was chiefly due to advantages derived from its 

 position. The original village offered security to fugitives from 

 Flanders, and their intelligence and skill laid the foundation of 

 manufactures and commerce. In 1313 William III. granted 

 t tli'' Baltic merchants exemption from tolls at Dort a privi- 

 lege afterwards extended to Amsterdam. The city joined the 

 Hanse League, and in 1342 it became necessary to enlarge its 

 boundaries. In 1368 the Swedish king assigned to Amsterdam 

 a district of the Isle of Schonen, as already noticed. 



War was entered upon in 1437 with the Hanso Towns, or 

 Osterlings. This resulted in an extension of the trade of the 

 Northern Netherlands, and in the acquisition of new commer- 

 cial advantages from Sweden, and soon after from Norway and 

 Denmark. In 1452 Amsterdam was burnt down, and a vast 

 quantity of merchandise destroyed. Twenty years sufficed to 

 restore the city, and to render it independent of the Hanse 

 protection, from which it severed itself in 1472. The trade 

 from this date increased even more rapidly than before. In 

 1482 tho inhabitants fortified their capital ; by 1500, commer- 

 cial relations had been established with Iceland and Russia. 

 Merchants from every European country met on its Exchange, 

 and it owned a mercantile fleet of two hundred sail. It was the 

 emporium of grain and foreign produce for Central Europe. 



Without any native timber, a forest was used in piles, and 

 large fleets were built and maintained. Surrounded by a barren 

 waste, the city was a vast granary, and a storehouse of tho 

 fruits of the earth. Fishermen, more expert than elsewhere, 

 made great hauls of herrings, yet no native-grown hemp could 

 be obtained for their nets, or sufficient iron for their fish-hooks. 

 No happy accidents or natural advantages favoured the city, yet 

 "tho sea not only bathed its walls, but entered among its 

 streets ; and the fleets of its merchantmen, as seen from the 

 ramparts, lay so crowded together, that vision was intercepted 

 by the thick forest of masta and yards." 



RECREATIVE SCIENCE. XXIII. 



IMPRESSION OF ACCIDENTAL OB COMPLEMENTARY COLOURS 



ON THE VISION SUMMARY OF THESE AND KINDRED 



ILLUSIONS. 



IT has been shown in a former paper that when the eye is 

 suddenly impressed with a very brilliant coloured light, after 

 it is extinguished the retina remains impressed with a colour 

 which is usually complementary to the one first observed. 



Complementary colours mean any two colours that will, when 

 combined together, form white light ; in fact, any two colours 

 which contain red, yellow, and blue. Thus a brilliant yellow 

 light would leave upon the eye the impression of lavender or 

 violet light, composed of rod and blue ; a green would leave 

 a red, and, vice versd, a black m&y impress the retina with 

 white ; and frequently where a dissolving view on a large screen 

 is suddenly darkened off or obscured, it leaves the dark out- 

 lines impressed for a few seconds on the retina of the eye as 

 distinct white lines, and this effect the writer has often 

 witnessed whilst observing the rapid dissolving away of the 

 picture of a map. 



The effect can be well shown by pasting some strips, say of 

 bright-red paper, in the form of a cross, on a sheet of white 

 cardboard. If the oxy-hydrogen light is projected from a 

 lantern, with condenser lenses, on to tho red cross, and the 

 spectator directed to stare at it steadily, on suddenly removing 

 the card with the red cross, and leaving another plain white 

 card behind, it will usually be noticed that nearly all those 



who are watching the experiment will nebum that the? Me a 

 green CTOM, faint of ooorne ; bat still quite afleientlr defined 

 to enable them to be rare that it u no. If, instead of the 

 red crom, green is need, red remain* vfefble, and black, a* 

 already stated, become* white. 



These effect* are described by Sir D. Brew*tr ae " aeeidenou 

 colour*," and he appean to regard them as synonymous with 

 the term already explained, namely, complementary colours. 

 Brewtter thus explains the phenomenon > "When the eye has 

 been for some time fixed on the red cross, the part of the 

 retina occupied by the red image U strongly *rrfoH. or, as it 

 were, deadened by its continued action. The sensibility to red 

 light will therefore be diminished ; and, consequently, when the 

 eye is turned from the red cross to the white card the Vadned 

 portion of the retina will be insensible to the red rays which 

 form part of the white light from the paper, and consequently 

 will see the paper of that colour which arises from all the rays 

 in the white light of the paper bat the red ; that is, of a bluish- 

 green colour, which is, therefore, the true complementary colour 

 of tho red cross." 



" When a black cross is placed on a white ground, the portion 

 of the retina on which the black image falls, in place of being 

 deadened, is protected, as it were, by the absence of light, 

 while all the surrounding parts of the retina, being excited by 

 the white light of the paper, will be deadened by its continual 

 action. Hence, when the eye is directed to the white card, it 

 will see a white cross corresponding to the black image on the 

 retina, so that the accidental colour of black is white." 



For the same reason, if a white cross is placed on a black 

 ground, and viewed steadfastly for some time, the eye will 

 always see a black cross; so that the accidental colour of 

 white is black. 



The same author remarks, " It is not, however, necessary 

 that the eye should be strongly impressed previously by some 

 coloured light, as the phenomena of accidental colours are 

 sometimes seen without it." 



Bre wstcr states that in order to see this class of phenomena he 

 found the following method the simplest and the best : " Having 

 lighted two candles, hold before one of them a piece of coloured 

 glass, suppose bright red, and remove the other candle to such 

 a distance that tho two shadows of any body formed upon a 

 piece of white paper may be equally dark. In this case one of 

 the shadows will be red, and the other green. With blue glass, 

 one of them will be blue and the other orange yellow, the one 

 being invariably the accidental, or complementary colour of the 

 other. The very same effect may be produced in daylight by 

 two holes in a window-shutter ; the one being covered with a 

 coloured glass, and the other transmitting the white light of 

 the sky." 



Here, however, it may be remarked, that the disturbing 

 cause is evidently the coloured light upon which the eye is 

 most likely to gaze intently first, and although the colourless 

 light is side by side, still it amounts to nothing more than 

 moving the eye suddenly from a red wafer to another part of 

 the paper where there is no red wafer. The eye is thus 

 impressed with green, and therefore it cannot be said, in the 

 cases quoted by Brewster, that the phenomena of accidental 

 colours are best seen when the eye has not been impressed 

 with any coloured object, because it is impressed by the red or 

 blue light obtained from the candle, or the rays of the sun 

 passing through glass coloured blue or red. What the writer 

 insists upon, is that in these cases it will always be found 

 that there is coloured light of some sort to set up the effect 

 of accidental colour. 



In the experiments devised and described by Mr. Rose, how- 

 ever, in a paper on " Persistence," it is clearly shown that with 

 no colour whatever to look upon, and only gazing on a white 

 card whilst tho stray light falling on it is gradually reduced 

 and restored, the white glare of light passes into various grada- 

 tions of coloured light. 



This very interesting experiment is thus described : " Aa 

 intensely white card is held before the eye, whilst a stronj 

 light falling on it is gradually reduced and restored. As tho 

 light is reduced the whiteness passes into yellow, orange, red, 

 and sometimes thence int blue. Whilst at other times colour: 

 intermediate between red and blue are apprehended, the gradual 

 reduction of the light brings up the colour by successive steps 

 and in reverse order to whiteness. All eyes, as might be ex- 



