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.

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