“Not unfrequently, after the conversion has taken place, the natural aspect of the object continues to intrude itself, sometimes suddenly, sometimes gradually, and for a longer or shorter interval, when the converse will again succeed it—as if the new visual impression could not at once counteract the previous results of recent experience. At last, however, the mind seems to accept the conversion without further hesitation; and after this process has once been completely gone through, the observer, on recurring to the same object, will not find it possible to see it in any other than its converted form, unless the interval should be long enough to have allowed him to forget its aspect.
“Vagaries, however, sometimes occur in these experiments of which it is difficult to give any certain explanation, but which would be probably found referable to the same general principle, if we were acquainted with all the conditions of its operation.”
The Multiplying-glass.
Still more extraordinary examples of the combining power of vision are to be found in the eyes of spiders and insects, more especially when we compare them with the work of man. If we take a common Multiplying-glass, such as is shown in the figure, and look at a flower or other object through it, we see the object repeated as many times as there are different foci of vision in the instrument.
Now, taking for example the eyes of a Spider, it would be natural to suppose that the same result would occur, especially as the foci of the eyes point in different directions. The left-hand figure in the illustration represents the eight eyes of one of our common Spiders, belonging to the genus Clubiona, which may be found in almost any outhouse, sitting in its curious web, and ready in a moment to run for safety into its silken tunnel.
It will be seen that the foci of all the eyes are in different directions, and so placed as to command a large radius. Observers have remarked that the eyes are placed in Spiders so as to suit their habits. “Those spiders,” writes Professor Owen, in his “Comparative Anatomy,” “which hide in tubes, or lurk in obscure retreats, either underground or in the holes or fissures of walls or rocks, from which they emerge only to seize a passing prey, have their eyes aggregated in a close group in the middle of the forehead, as in the Bird-spider, the Clotho, &c.
“The spiders which inhabit short tubes, terminated by a large web, exposed to the open air, have the eyes separated and more spread upon the front of the cephalothorax.
“Those spiders which rest in the centre of a free web, along which they frequently traverse, have the eyes supported on slight prominences, which permit a greater divergence of their axis; this structure is well remarked in the genus Thomisa, the species of which live in ambuscade in flowers.