3i! 



SCIENTIFIC NEW^S. 



[April 6, 1888. 



way under the weight of the body. The great toe is 

 large and prominent, decidedly longer than the second 

 toe. The depth of the impression is most marked at the 

 anterior part of the foot — the habitual indication of a 

 Tigorous pedestrian. 



Dr. Flint has lound other human foot-marks in 

 different parts of Nicaragua. All testify that this country 

 was peopled at very remote epochs, which, however, 

 cannot be determined. The eruptions, the floods, the 

 upheavals and displacements which they involve in 

 volcanic regions are in such regions both very frequent 

 and very irregular in their duration and their intensity. 

 To this must be added the luxuriant tropical vegetation, 

 which in Nicaragua rapidly destroys all traces of man. 

 As for the mastodon it is in no wise characteristic of any 

 special epoch, since it has survived in America down to 

 historical times. Recently there has been found at 

 Concordia a complete skeleton of this animal in a pool 

 paved with stone slabs. The shells found in the sands 

 underlying the stratum containing the footprints are 

 ascribed to the quaternary epoch. Consequently the 

 footprints themselves cannot belong to an earlier epoch. 



CHROMATOPHORES. 



'T'AKE two live trout or speckled gold-fish, similar in 

 size and colour. Have ready two earthenware 

 bowls, one white inside, the other completely blackened 

 with paint. Fill these with water, and put a fish in 

 each. Under the action of light the colour of the fishes 

 will gradually change. After ten minutes or a quarter 

 of an hour a marked difference will be observed, if both 

 the fishes are placed in the same bowl. The fish which 

 has been kept on the dark ground will be visibly darker 

 than the other. The experiment is best made by day- 

 light, but gaslight will do. It is easy to vary the con- 

 ditions, so as to prove beyonJ all doubt that change in 

 the amount of light reflected from the bottom produces a 

 rapid and conspicuous change in the colour of the fish. 

 Frogs and shore-fishes will be found to give similar re- 

 sults. The advantage to the fish or frog of this power of 

 accommodation is obvious enough. The animal escapes 

 detection by protective resemblance to its surroundings. 

 Many naturalists have remarked that in the desert nearly 

 all animals are sand-coloured ; that many of the animals 

 of polar or alpine districts are white as snow or speckled 

 with white. Some inhabitants of sub-arctic regions change 

 to white as winter comes on. The sloth in the dingy 

 green surroundings of a dense forest assumes a grey- 

 green colour. Flat fishes are usually sand-coloured 

 above, so that looking down upon them we can hardly 

 distinguish them from the sand upon which they lie, but 

 they are white beneath, so that when looked at from 

 below as they flutter to the surface they harmonise very 

 well with the white light pouring down from the sky. 



The mechanism of accommodation is very diverse. 

 Sometimes it depends upon the pigment contained in the 

 hair or feathers. The sloth disguises himself by means 

 of a crop of microscopic green algae, which find a coign of 

 vantage in deep channels sunk in his coarse hair. 

 Aquatic animals usually have recourse to the pigment- 

 cells of the skin as a means of assimilating themselves 

 to the objects around. Any observer, moderately skilled 

 ill the use of the microscope, can see for himself the pig- 

 ment-cells of a frog or a fish. A little bit of the skin 

 must be examined both in surface-view and in section. 



The cells will be readily made out as blotches of black, 

 yellow, and other colours lying in the skin. They 

 are what are technically called " chromatophores," a 

 kind of connective-tissue corpuscles, straggling masses 

 of protoplasm with wide meshes, and lodging drops of 

 the oily pigment which they secrete. The protoplasm 

 gives evidence of irritability. When stimulated in a 

 particular way, it contracts to a spherical shape, gather- 

 ing up all the contained pigment into the smallest possible 

 space. Another kind or degree of stimulus causes it to 

 dilate into a stellate figure, with a central blot, and long 

 rays ending in knobs. Hence the pigment, though con- 

 stant in amount, produces very different effects. If con- 

 tracted into spherical drops it intercepts very little light ; 

 if dilated, it may suffuse the whole surface.. 



We may next inquire whether the mechanism depends 

 upon the immediate stimulus of the incident light. Do 

 the corpuscles contract or dilate automatically, according 

 to the intensity or amount of the light which falls upon 

 them ? A little reflection shows that such a property 

 would be quite useless. It is not the light which falls 

 upon the body, but the light reflected from surrounding 

 objects which has to be matched. Between this reflected 

 light and the pigment-cells the opaque body of the animal 

 is usually interposed, and the cells cannot accommodate 

 themselves to a condition of which they are, so to speak, 

 unaware. Hence the pigment apparatus may be sup- 

 posed to stand in need of a percipient and regulating 

 mechanism outside itself. Experiment shows that such 

 a mechanism exists in the eye and the cutaneous nerves. 

 If the fish be blinded, or if a certain branch of the tenth 

 pair of nerves be cut, the power of accommodation is 

 immediately lost. The eye judges of the colour of the 

 ground, the central nervous system receives the impres- 

 sion communicated from the eye, and regulates through 

 the cutaneous nerves the behaviour of the pigment-cells. 

 There is every reason to suppose that the action is alto- 

 gether unconscious and involuntary. 



If we call up to our recollection the varying hues of 

 the chamseleon or cuttle-fish, the adaptation of the colours 

 of ticks to the parti-coloured scales of snakes, the splash- 

 ing and speckling of certain birds'-eggs in imitation of 

 the lime-sprinkled rocks about the sea-side breeding- 

 place, and the infinite varieties of colour which depend 

 not upon pigment but iridescence, we see what a great 

 part protective colouring plays in nature. Changing 

 circumstances and the capricious differences of individual 

 animals furnish an endless series of natural experiments, 

 and every successful experiment is embodied in a per- 

 manent modification of the protective mechanism. 



The White Lead Industry of Austria. — Four different 

 grades of white lead are manufactured in Austria, Kremser 

 white, Venetian white, Hamburg white, and Dutch white. 

 The first bears this name because at some remote period an 

 excellent quality of white lead was manufactured at Krems, 

 an ancient city of Lower Austria, but none has been produced 

 there for at least 150 years ; but the best quality white lead 

 still bears the name Kremserweiss. The same is the case 

 with the Venetian white ; the name, like Venetian red, has 

 survived, although the industry has been dead for centuries. 

 Kremser white contains a trace only of indigo, Venetian 

 white contains equal parts of lead and baryta, or blanc-fixe, 

 Hamburg white contains one part of lead to two of baryta, 

 while in the Dutch white there are three parts of baryta to 

 one of lead. The raw material is obtained from the lead 

 mines, the most important of which are situated in Villach 

 and Bleiberg, in Carinthia. 



