“It is the boast of science to have been able to trace so far the refined contrivances of this most admirable organ, not its shame to find something still concealed from scrutiny; for, however anatomists may differ on points of structure, or physiologists dispute on modes of action, there is that in what we _do_ understand of the formation of the eye, so similar, and yet so infinitely superior to a product of human ingenuity; such thought, such care, such refinement, such advantage taken of the properties of natural agents used as mere instruments for accomplishing a given end, as force upon us a conviction of deliberate choice and premeditated design, more strongly, perhaps, than any single contrivance to be found whether in art or nature, and renders its study an object of the greatest interest.”[107] Has the reader ever asked himself why it is, having two eyes, and consequently two pictures produced upon the tablets of vision, that we see only one object? According to the law of visible direction, all the rays passing through the crystalline lenses converge to one point upon the retina,--and as the two images are coincident and nearly identical, they can only produce the sensation of one upon the brain. When we look at any round object, as the ornamented moderator lamp before us, first with one eye, and then with the other, we discover that, with the right eye, we see most of the right-hand side of the lamp, and with the left eye more of the left-hand side. These two images are combined, and we see an object which we know to be round. This is illustrated in a most interesting manner by the little optical instrument, _the Stereoscope_. It consists either of two mirrors placed each at an angle of 45°, or of two semi-lenses turned with their curved sides towards each other. To view its phenomena, two pictures are obtained by the camera obscura on photographic paper of any object in two positions, corresponding with the conditions of viewing it with the two eyes. By the mirrors or the lenses these dissimilar pictures are combined within the eye, and the vision of an actually solid object is produced from the pictures represented on a plane surface. Hence the name of the instrument; which signifies, _Solid I see_. Analogy is often of great value in indicating the direction in which to seek for a truth; but analogical evidence, unless where the resemblance is very striking, should be received with caution. Mankind are so ready to leap to conclusions without the labour necessary for a faithful elucidation of the truth, that too often a few points of resemblance are seized upon, and an inference is drawn which is calculated to mislead.