To effect this, we must carefully cut off this delicate structure, and so prepare it that we may employ upon it the first of a series of our highest powers. The result of that examination is given here.[5] You see that the whole organ has a distinct form and border, and that its carefully carved surface gives origin to wheel-like areolæ which form the bases of delicate hairs. The function of this organ is really unknown. It is known from its position as the _pygidium_; and from the extreme sensitiveness of the hairs to the slightest aerial movement, may be a tactile organ warning of the approach of enemies; the eyes have no power to see. But we have not reached the ultimate accessible structure of this organ. If we place a portion of the surface under one of the finest of our most powerful lenses, this will be the result.[6] Now, without discussing the real optical or anatomical value of this result as it stands, what I desire to remind you of is: 1. The natural size of the flea. 2. The increase of knowledge gained by its general enlargement. 3. The relation in size between the flea and its pygidium. 4. The manner in which our lenses reveal its structure, not merely amplify its form. [Footnote 5: The pygidium of the flea, very highly magnified, was here shown.] [Footnote 6: An illustration of the pygidium structure seen with one-thirty-fifth immersion was given.] Now with these simple and yet needful preliminaries you will be able to follow me in a careful study of the least, the very lowliest and smallest, of all living things. It lies on the very verge of our present powers of optical aid, and what we know concerning it will convince you that we are prepared with competent skill to attack the problem of the life-histories of the smallest living forms. The group to which the subject of our present study belongs is the bacteria. They are primarily staff-like organisms of extreme minuteness, but may be straight, or bent, or curved, or spiral, or twisted rods. This entire projection is drawn on glass, with _camera lucida_, each object being magnified 2,000 diameters, that is to say, 4,000,000 of times in area. Yet the entire drawing is made upon an area of not quite 3 inches in diameter, and afterward projected here. The objects therefore are all equally magnified, and their relative sizes may be seen. The giant of the series is known as _Spirillum volutans;_ and you will see that the representative species given become less and less in size until we reach the smallest of all the definite forms, and known to science as _Bacterium termo_.