Now within given limits this organism varies in size, but if a fair average be taken its size is such that 50,000,000 laid in order would only fill the hundredth of a cubic inch. Now the majority of these forms _move_ with rapidity and grace in the fluids they inhabit. But how? By what means? By looking at the largest form of this group, you will see that it is provided with two delicate fibers, one at each end. Ehrenberg and others strongly suspected their existence, and we were enabled, with more perfect lenses, to _demonstrate_ their presence some twelve years ago. They are actually the swimming organs of this Spirillum. The fluid is lashed rhythmically by these fibers, and a spiral movement of the utmost grace results. Then do the intermediate forms that move also possess these flagella, and does this least form in nature, viz., _Bacterium termo_, accomplish its bounding and rebounding movements in the same way? Yes! by a series of resolute efforts, in using a new battery of lenses--the finest that at that time had ever been put into the hands of man--I was enabled to show in succession that each motile form of Bacterium up to _B. lineola_ accomplished its movements by fibers or flagella; and that in the act of self-division, constantly taking place, a new fiber was drawn out for each half before separation. But the point of difficulty was _B. termo_. The demonstration of its flagella was a task of difficulty which only patient purpose could conquer. But by the use of our new lenses, and special illumination we--my colleague and I--were enabled to demonstrate clearly a flagellum at each end of this least of living organisms, as you see, and by the rapid lashing of the fluid, alternately or together, with these flagella, the powerful, rapid, and graceful movements of this smallest known living thing are accomplished. Of course these fibers are inconceivably fine--indeed for this very reason it was desirable, if possible, to _measure_ it, to discover its actual thickness. We all know that, both for the telescope and the microscope, beautiful apparatus are made for measuring minute magnified details. But unfortunately no instrument manufactured was delicate enough to measure _directly_ this fiber. If it were measured it must be by an indirect progress, which I accomplished thus: The diameter of the body of _B. termo_, _i.e._, from; side to side, may in different forms vary from the 1/20000 to the 1/50000 of an inch. _That_ is a measurement which we may easily make directly with a micrometer. Haying ascertained this, I determined to discover the ratio of thickness between the body of the Bacterium and its flagellum--that is to say, to discover how many of the flagella laid side by side would make up the width of the body.