drawing of the magnified image was made was extremely small—­a mere white speck in the strongest light upon a black ground.  But you observe it is not a speck of form merely enlarged.  It is not merely beauty of outline made bigger.  But there is—­as in the delicate group you now see—­a perfect opening up of otherwise absolutely invisible details.  We may strengthen this evidence in favor of the analytical power of our higher lenses by one more familiar example, and then advance to the most striking illustration of this power which our most perfect and powerful lenses can afford.  I fear that may be taking too much for granted to assume that every one in an audience like this has seen a human flea!  Most, however, will have a dim recollection or suggestive instinct as to its size in nature.  Nothing striking is revealed by this amount of magnification excepting the existence of breathing pores or spiracles along the scale armor of its body.  But there is a trace of structure in the terminal ring of the exo-skeleton which we cannot clearly define, and of which we may desire to know more.  This can be done only by the use of far higher powers.

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 areolae 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.]