AIDS TO VISION.
HELMHOLTZ was fond of saying, in proof of the imper- fections of the human eye, that he would unhesitatingly reject any piece of work sent to him by an instrument-maker which should prove to be equally full of defects. It is need- less to accumulate illustrations of a fact which is familiar to all scientific workers, and which the wisdom of the ages has vainly tried to combat in the aphorism that "seeing is believing." To give only one instance of the untrustworthi- ness of the human eye, it is still debated whether the so-called canals which the telescope seems to reveal on the surface of Mars are not an optical delusion, due to the inveterate tendency of the eye to arrange separate and unconnected markings into a linear pattern. Any one who has watched a really good conjurer is well aware that seeing is not always believing : we see him quite distinctly take a coin out of his left eye and convert a playing-card into a white rabbit ; but all the same we do not believe that he does it. Yet it is almost entirely through the medium of this imperfect and deceptive instrument that we are obliged to prosecute those researches into the nature of things which, under the generic name of science, have done so much to change the conditions of life, and the outlook of mankind upon both the material and the spiritual universe. Fortu- nately, we are able to check the messages which the eye sends to the brain by the indications of other senses, as well as to supplement its native weakness by instrumental aids. Only those who have studied the singular aberrations into which a false notion of religion can lead the spirit of mankind will readily believe that, as Professor Gotch reminded us in a recent Oxford lecture, an attempt was made in the early days of these inventions to decry their use. A certain Somerset vicar made himself notorious by the vigour with which, in the seventeenth century, he inveighed against the use of the newly invented optic glasses, since they perverted vision, and made all things appear in an unnatural, and therefore a false, light. Microscopes and telescopes, with their array of lenses, he declared to be impostors, since a man could not see so well with two pair of spectacles as with one. Some asserted it to be sinful to assist the eyes, which were adapted by Providence to the capacity of the individual, whether good, bad, or indifferent. "It was argued that society at large would become demoralised by the use of spectacles ; they would give one man an unfair advantage over his fellow, and every man an unfair advantage over every woman, who could not be expected, on aesthetic and intellectual grounds, to adopt the practice." We seem to have heard very similar reasoning on another subject quite recently. Fortunately the common-sense of mankind is usually victorious over such reasoning. It is difficult to argue people out of the use of notoriously advantageous inventions. To persuade the scientific investigator to throw away his aids to vision would be as bard as to persuade the artillerist to abandon his big guns for the bows and arrows which Mr. Herbert Paul recently discovered to be more natural to man.
Perhaps it is not often realised how entirely a great part of our knowledge is dependent upon the instrumental aids to vision. To take a single instance, modern bacteriology, which has already thrown such a flood of light upon the incidence and prevention of disease, would be impossible without the microscope. It is not without significance that the latest handbook to the use of the microscope, Sir Almroth E. Wright's " Principles of Microscopy," which has just been published by Messrs. Constable (21s. net), is the work, not of an optician, but of a pathologist. This admirable and suggestive work is too rigorously technical for review in our columns, but it serves us to take account of the remarkable advances which have been made in the construction and use of microscopes during the two and a half centuries which have elapsed since Leeuwenhoek first succeeded in grinding lenses that were able to reveal the hidden secrets of Nature. There is good reason to suppose that the first principles of microscopy were known in very early times. Layard dug up a convex lens of rock-crystal among the ruins of Nineveh, and it is practically certain that the exquisite gems of the ancient world could not have been cut in their minute detail without some such aid. But it was only with the advent of Leeuwenhoek's genius that men began to realise how the microscope, as Berkeley puts it, "brings us, as it were, into a new world. It presents us with a new scene of visible objects, quite different from what we behold with the naked eye." Berkeley, indeed, thought this a doubtful advantage, because the structural details revealed by the microscope were too small to have any connexion with tangible objects, "whereby we are taught to foresee what will ensue upon the approach or application of distant objects to the parts of our own body, which much conduceth to its preservation." But for once the acute philosopher of Cloyne was wrong. The researches of Spallanzani and his successors, culminating in the epoch-making work of Pasteur, have taught us that minute forms of life which we only know through the microscope have a most direct bearing upon our health and well- being ; a knowledge of them does, in short, very distinctly conduce to our preservation. Sir Almroth Wright reminds us that microscopical research in biology, on which modern theories of life and hygiene are so entirely dependent, is oddly connected with "the fact that particular dyes are fast only upon particular textures—a fact which sooner or later enforces itself upon the attention of every purchaser of hosiery." It is not so much improvements in microscopes as in their use which have made them such potent aids to the biologist and bacteriologist. The discovery that certain stains or dyes will adhere to one set of tissues or organisms, whilst having no affinity with others, has enabled us to make great advances in identifying the various structures in a microscopic section, and work on these lines depends mainly upon "the invention and exploitation of new methods of differential staining." As an instrument of biological research the modern microscope is sufficiently powerful. A good instrument is now capable of showing an object which is only one-hundred-thousandth part of an inch in diameter: and this is not far from the theoretical mimimum visibile, which depends on the wave-length of light. Mr. Gordon's remarkable discovery of the possibilities of the so- called tandem microscope, which is described in the last chapter of Sir Almroth Wright's book, indicates that we may before long obtain a still higher degree of resolution; but the imperfections of the human eye render it at least doubtful whether it will be able to use more powerful microscopes to any advantage. Poor humanity is at fault again.
We are led from the infinitely little to the infinitely great by another book which lies before us, Mr. James Baikie's "Through the Telescope" (A. and C. Black, 5s. net). The telescope has done as much as the microscope for man's practical mastery of the world. Without it, navigation would still be an empirical art, and we should have no certain rules by which the sailor might steer across the great oceans with every expectation of a safe landfall. Mr. Baikie's chief concern in this lucid and agreeable book is with those celestial objects which can be profitably studied through such a telescope as is within the reach of the average amateur. But in his introductory chapter he gives a succinct account of the evolu- tion of the telescope. The world is still in doubt as to its actual inventor, but there is a consensus of opinion that the man who first showed what marvels of discovery lay in the simple-seeming optic tube was "starry Galileo." His telescope was a very humble instrument—the sort of thing that any intelligent schoolboy can now make unto himself at an expenditure of a very few shillings—yet Proctor was right in saying that "if we regard the absolute importance of the discoveries effected by different telescopes, few, perhaps, will rank higher than the little tube now lying in the Tribune of Galileo at Florence." The popular idea of a telescope is that it is merely used for making out the details of heavenly objects, and that an increase in size and definition is the sole desideratum of astronomers. As a matter of fact, this is only one, and perhaps not the most important, use of the astronomical telescope. The whole science of navigation depends for its accuracy on the adaptation of the telescope to find the exact direction of a star or planet, and a comparatively small telescope does this work better than any of the great modern instruments. A thirteen- inch refractor was selected for the purposes of the inter- national photographic survey of the heavens, in which some millions of stars, of which the great majority are for ever invisible to the naked eye, are mapped. The spectroscope, too, which has told such astonishing things about the con- stitution and motions of far-distant star-systems, does not need a very large telescope to do its work. Probably no further attempt will be made to construct an instrument as large as Lord Ruse's six-foot reflector. We shall never be able to make a telescope powerful enough to resolve with any certainty the interesting question as to the habitability of the planets, and messages from Mars, if we ever receive them, will come by some kind of wireless telegraphy or thought transference. But there is some cause for gratification at the achievements of human intelligence, thus working with the aid of a few pieces of glass and metal.
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