The science of life : fully illustrated in tone and line and including many diagrams

THE LOWLY AND MINUTE

§ 8 The Smallest Living Things

The invention and gradual improvement of the compound microscope have led us stage by stage into a world of tiny wonders, a world that becomes more and more mysterious as one proceeds to smaller creatures. In the first excitement of this discovery it was thought that our power of magnification was, in theory at least, unlimited. It was thought that one could go on combining lenses for ever, that if only one could make lenses without aberrations of line and colour there would be no secret of Nature, however minute, that one could not magnify and make visible. But, unhappily, that is very far from true. There is a limit to the smallness of the things which can be seen, a limit imposed not by any fault of the lens-grinders but by a property of light itself. We know that we shall never be able to see distinctly anything less than one four-thousandth of a millimetre across (one hundred-thousandth of an inch), however powerful our lenses may be, because of the unalterable wave-length of visible light. Smaller objects can indeed be made perceptible by means of optical tricks, but they cannot be examined ; they appear as spots or blurs without definable shape. Now, that limit has already been reached. -Microscopes have been made which clearly reveal objects of that size, and it has been proved that there are living things even smaller than this.

It may sound paradoxical to say that we are sure of the existence of living things which cannot possibly be seen, but, as a matter of fact, our belief rests on solid ground. But before we go into this matter we may pause for a while and define a new word. For it is inconvenient to measure these minute distances in such comparatively vast units as millimetres or inches; it is like describing the dimensions of a postage stamp in fractions of a mile. The microscopist, in studying these matters, uses a measure of his own invention called a micron (usually written ); a micron is one-thousandth part of a millimetre, or about one twenty-five thousandth of an inch. To the unaided eye it is an invisibly small distance. The smallest object that we can observe with any microscope is about a quarter of a micron across—and only the very best instruments can take us as far as this. Most bacteria come pretty near this limit; the diameter of a coccus is usually somewhere about a micron. And now for the evidence on which our

belief rests.

The argument is very simple. Suppose we pass a handful of coarse sand through a sieve whose pores are a twentieth of an inch in diameter ; we know that none of the grains that are bigger than this can get through. It is possible to get filters whose pores are about a quarter of a micron in diameter—they are made of unglazed porcelain or of similar material—and we know that if we pass a fluid through such a filter anything bigger than a quarter of a micron across will be held back. This is the limit of microscopic vision. Therefore, we may infer that nothing visible through the microscope will be able to pass such a filter —an inference that can be tested by direct experiment. Now there are several disorders that can be transmitted to healthy subjects by injecting them with fluids that have passed through such a filter. Footand-mouth disease was the first to be discovered. The characteristic feature of this cattle scourge is the appearance of sore blisters, especially on the hoofs and lips. If a little of the fluid from these blisters be passed through the finest porcelain filter, so that all traces of visible bacteria are eliminated from it, it will, nevertheless, infect a healthy beast if it is injected into its blood ; moreover, a third beast can be infected from the blisters of the second, and so on. Evidently there must be a living thing in this fluid, a thing that can multiply and proliferate. If it was simply a poisonous substance its effects would become weaker and weaker as it was transferred through successive beasts, but this does not occur. Like a bacterial infection, the invisible creature proliferates inside its host.

A living thing of this kind, too small to be seen with the highest powers of the microscope and passing even a porcelain filter, is called an “‘ ultramicroscopic ” or “ filterpassing’ organism; since all the known kinds cause diseases they are also spoken of as viruses. There are other diseases with similar causes. Smallpox, typhus fever, trench fever, yellow fever, measles, mumps, the distemper of dogs, and a number of vegetable afflictions causing the leaves of potatoes, beans, turnips, and the like to come out in white spots—these are examples. Perhaps the common cold is another.

in these unmagnifiable creatures

1 Strictly speaking, the term “‘ ultramicroscopic ” and “‘ filter-passing ”’ are not synonymous. There are slender, serpentine creatures large enough to be seen through the microscope, yet able to twist and wriggle through a porcelain filter.

185