I proceeded thus: This is a complicated microscope placed on a tripod, so arranged that it may be conveniently worked upright. There is a special instrument for centering and illuminating. On the stage of the instrument, the Bacterium with its flagellum in distinct focus is placed. Instead of the simple eyepiece, _camera lucida_ is placed upon it. This instrument is so constructed that it appears to throw the image of the object upon the white sheet of paper on the small table at the right hand where the drawing is made, at the, same time that it enables the same eye to see the pencil and the right hand. In this way I made a careful drawing of _B. termo_ and its flagellum, magnified 5,000 diameters. Here is a projection of the drawing made. But I subsequently avoided paper, and used under the camera most carefully prepared surface of ground glass. When the drawing was made I placed on the drawing a drop of Canada balsam, and covered it with a circle of thin glass, just like any other microscopic mounted object. This is a micro-slide so prepared. Now you can see that I only have to lay this on the stage of a microscope, make it an object for a low power, and use a screw micrometer to find how many flagella go to the making of a body. The result is given in the figure; you see that ten flagella would fill the area occupied by the diameter of the body. In the case chosen the body was the 1/20,400 of an inch wide, and therefore, when divided by ten, gave for the flagellum a thickness of the 1/204,000 of an English inch. In the end I made fifty separate drawings with four separate lenses. I averaged the result in each fifty, and then took the average of the total of 200, and the mean value of the width of the flagellum was the 1/204,700 of an English inch. It will be seen, then, that we are possessed of instruments which, when competently used, will enable us to study the life-histories of the putrefactive organisms, although they are the minutest forms of life. I have stated that they were the inevitable accompaniments of putrescence and decay. You learned from a previous illustration the general appearance of the Bacteria; they are the earliest to appear whenever putrefaction shows itself. In fact the pioneer is this--the ubiquitous _Bacterium termo._ The order of succession of the other forms is by no means certain. But whenever a high stage of decomposition is reached, a group of forms represented by these three will swarm the fluid. These are the Monads, they are strictly putrefactive organisms, they are midway in size between the least and largest Bacteria, and are, from their form and other conditions, more amenable to research, and twelve years ago I resolved, with the highest power lenses and considerable practice in their use, to attack the problem of their origin; whether as physical products of the not-living, or as the natural progeny of parents.