Page:Hardwicke's Science-Gossip - Volume 1.pdf/24

8 which again grow other fronds, and so on ad infinitum. The method of growth in the three other species is precisely similar, and can be clearly seen in all under a magnifying power.

Had it been the intention of this paper to embrace any account of other living organisms in the aquarium, a description might have been given how the tadpoles were continually biting off the rootlets of the Duckweeds; how the great beetles seized the tadpoles, and bit their tails off, so that they could not rise to the surface; how the tadpoles again in their turn attached themselves, six or eight at a time, to the sides of the carp, where they hang for days together, going round and round with the fish, sucking their juices, and becoming veritable parasites; how the fish were continually gulping down fronds of duckweed, and as quickly disgorging them; how, when the tadpoles were dead, because the aquarium and the beetles would not let them become frogs, the snails were the objects of the beetles' fury; and how, at length, the beetles gave themselves up to unmitigated cannibalism, and "did each other eat." Such a description would doubtless be instructive and entertaining enough to induce us to return to it on some future occasion. W. G. S.

HY objects under the microscope appear so much larger than when seen by the naked eye, is an inquiry that would naturally suggest itself to the mind of an intelligent lad or adult, who for the first time in his life peered through the tube of his friend's microscope. The answer to such an inquiry will, we believe, reveal to us the principle on which the microscope itself is constructed. A mere speck, undistinguishable either in form or colour, is placed before us. Our friend transfers it to the stage of his microscope, adjusts the tube, and invites us to take a peep. We do so. Astonishment seizes upon us. Every part of that object is clearly mapped out; every member of its body duly displayed, and even a portion of its internal organization is distinctly revealed. We depart to our homes mentally asking, By what means such an addition of power is obtained for the eye?

In what, then, does magnitude consist, that at one time an object should appear large, and at another time small? Let us endeavour to unravel this mystery.

It will readily be admitted that we judge of size by comparison—we compare one object with another—and thus form an idea of the extent it occupies in space. But had we nothing with which we could make the comparison, it would be impossible to judge of size at all.

It will be found that the apparent size of an object varies with the distance it may be from the eye of the spectator. Thus, a balloon at a great altitude, or a castle at a distance, both appear vastly diminished in bulk by reason of their remoteness; and had we no other means of judging their magnitude than those furnished by the eye, we should unhesitatingly believe the first to be no larger than a boy's top, and the latter a fitting residence for a child's twopenny doll. Even the sun himself, under similar circumstances, would be dwarfed to the size of the crown of one's hat. Fortunately we have other means at our disposal for correcting our visual impressions.

If the aëronaut at our bidding suddenly quitted the clouds and descended at our feet, we should observe his balloon gradually expand in every direction until it assumed the dimensions of a house. A small portion of the balloon contained within one of the lozenge-shaped meshes of the netting would appear at this short space even larger than the entire machine when seen from a distance. So also with the castle. If we travel towards it, we shall perceive it looming out larger and larger, as the distance decreases between it and ourselves. At last we stand beneath its walls. We now see but a very small portion of the building. It has so rapidly increased in size that a single stone covers with its image a larger space on the retina of the eye than the whole structure when seen afar off. If, then, the magnitude of an object depends on the distance the eye of the spectator may be from it, it ought to follow that the nearer the eye the larger the appearance of the object; and this we think will be found the be the case. There is, however, a limit to the eye in this direction. In an ordinary healthy eye ten inches is considered to be the distance at which small objects can be most distinctly seen, and this may be termed its natural focus. The eye is so constructed that it cannot accommodate itself to a shorter distance than its natural focus