Feb. 1, 1886.] 



KNOAVLEDGE 



ii; 



on a tightly-stretched T\hite sheet, or large sheet of card- 

 board. Under these circumstances it will appear grey. 

 Now, though, let us light a candle. This will also illu- 

 minate our white background, of course, with a yellowi.sli 

 light, and will cast a second shadow of our rod, which 

 should be made to fall a short distance either to the right 

 or left of our shadow number one. Care should further 

 be taken by varying, if necessary, the size of the hole 

 through which the daylight comes, to render the shadows 

 equal in intensity. Under these circumstances, the rod 

 cutting off the light of the candle from that part of the wall 

 or screen upon which its shadow falls (and which is thus 

 illuminated by daylight alone) should, theoretically, leave 

 it white; but, owing to the effect of contrast, it will 

 apiiear quite perceptibly blue ; what we may call the 

 dr^jlight shadow of the rod seeming to be of a yellowish 

 colour, as might be expected from the fact that it is 

 illuminated by the yellow candle-flame. A very re- 

 markable addition to this experiment was devised by 

 Helmholtz, who viewed the inner edges of the two 

 shadows — and, of course, the intei-mediate strip of the 

 illuminated field — through a tube blackened internally, 

 and gazed at them until the second shadow exhibited its 

 most pronounced blue tint. Then he moved the tube until 

 this shadow filled the field of view, when, c«rious to 

 relate, although the yellow field, to the original contrast 

 with which the seemingly blue colour was due, had 

 now disappeared, the opening of the tube still seemed 

 to be filled with bluish light. Nay, what appears still 

 more surpri.i-ing, upon blowing the candle out, the 

 impression of blueness remained, f.lbeit it vanished 

 r.t once on removing the tube and looking straight at 

 the .screen. Here, evidently, the illusion must be in our 

 own judgment, irrespectively altogether of any specific 

 action on tho nervous fibrilbe of the retina. Vari;itions 

 of this curious and really startling effect may be made 

 by having two holts in the shutter with which wo darken 

 the room, and covering them with differently-coloured 

 glasses or sheets of coloured gelatine. Another experi- 

 ment, made with even less trouble than the one just 

 described, was contrived by Mayer, who upon a sheet of 

 bright green pajier placed a small rectangle of grey paj.er. 

 Gazing at this the green will look green and the grey 

 grey, the effect of coutr.ast being almost imperceptible. 

 If, though, we cover the whole arrangement with a thin 

 piece of tis.sue- paper, the green is notably diluted and 

 weakened, and our grey slip of paper at once puts on a 

 distinctly reddish hue. From this we derive a rule of 

 considerable value to the artist — viz, that pale tints in 

 juxta[iosition exhibit the effect of contrast much more 

 vividly than absolutely pure, strong ones do. Further- 

 more, it will be f.yund that if we place in contact two 

 rectangles of paper coloiu-ed identicallj- save that one 

 is pale and the other deep in tint, the pale slip will look 

 paler, and the darker slip darker than when they are 

 viewed apart. The best way to show this is to cut two 

 such pieces out of each sheet ; lay one from each piece 

 together, and the remaining two, at some distance apart, 

 upon a table, and look at them. The effect of proximity 

 in seemingl}' altering their hues will strike the eye at 

 once. Finally, the result produced by the mere contiguity 

 of differently-shaded surfaces may be instructively show n 

 by colouring a series of cards with absolutely flat washes 

 of Indian-ink or lamp-black, each one darker than its pre- 

 decessor. Laying these one over another, they almost 

 suggest the idea of a fluted column, so much darker do 

 the light ones appear where they are in contact with 

 lighter ones .stiU, and vice-versd. This effect is seen on a 

 grand scale in North Wales and other mountainous 



countries, where the landscape so often contains range 

 after range of mountains up to the horizon. The 

 .slightest attention will show that the lower pai-ts 

 of any one of these ranges invariably seems lighter than 

 the upper part of that immediately in front of it ; and it 

 must be always so represented in a sketch, if an accurate 

 reproduction of the natural effect is aimed at. In fact, 

 it will be observed that the sharp dark edge of any 

 visible object lightens, quite notably, that part of a more 

 distant one immediatelj' behind it. The effects of 

 contrast further produced by tho superposition of colours 

 upon black, white, and grey backgrounds, or by contact 

 with masses of them, will be best learned from a series 

 of experiments by the student himself. A very little 

 thought will enable him to apply the principles we have 

 been endeavouring to make clear, to the explanation of 

 the phenomena which he will observe. 



NOTES ON EARTHQUAKES, 

 By Richard A. rROcTOK. 

 THE EARTHQUAKE OF CALABRIA. 

 O e-'rthquake has ever happened, the cir- 

 cumstances attending which have been so 

 carefiilly noted as in the case of the carth- 

 i|uake of Calabria in 1783. This cele- 

 brated earthquake began in February, 

 1783, and lasted until the end of 1786. 

 The fir.st shock threw down, "in two 

 minutes, nearly every house in all the citit.s, towns, 

 and villages, from the western flanks of the Apen- 

 nines in Calabria Ultra to Messina in Sicily, and con- 

 vulsed the whole country." The second took place seven 

 weeks later, otid was scarcely less violent. Sir Charles 

 Lyell mentions that " the great granite chain which 

 jwsses through Calabria from north to south, and attains 

 the height of m;iny thousand feet, was shaken but slightly 

 by the first shock, but rudely by those which followed." 



The manner in which a large extent of country was 

 permanently affected by this earthquake is very well 

 worth noticing,- as affording an excellent illustration of 

 the mode in which earth-waves travel. 



The Apennines are formed for the most part of 

 massive and hard granite, with steep inclines, upon the 

 base of which lie those strata of sand and clay which 

 form the Calabrian plains. These plains are usually 

 level, but are intersected in places by narrow valleys and 

 ravines whose sides are almost vertical. The eff'ect of 

 the earthquake was to shake cloien those parts of the 

 Calabrian plains which border on the granite backbone 

 forming the Apennine range. The soil " slid over the 

 solid and inclined nucleus, and descended somewhat 

 lower," .says Lyell, "leaving almost uninterruptedly from 

 St. George to beyond St. Christina— a distance of fnmi 

 nine to ten miles — a chasm between the solid granite 

 nucleus and the sandy soil. Many lands slipiiing thus 

 were carried to a considerable distance from their former 

 position, so as entirely to cover others; and disputes 

 arose as to whom the jiroperty which had thus .shifted its 

 place .should belong to." 



The whole of the country over which the effects of 

 the great shocks extended was at times heaved simul- 

 taneously, like an angry sea, and sensations resembling 

 sea-sickness were experienced by many of the inhabitants. 

 Those who have watched the sky from the deck of a 

 sea-tossed .ship will have noticed that the drifting clouds 

 seem at times to be arrested in their motion: it is in 

 reality the .ship which is moving for the moment in the 



