The Rev. W. H. Dallinger lately delivered a lecture at the Royal Institution, descriptive of the recent researches of Dr. Drysdale and himself. The object of the lecture was mainly to explain the method of research which had been employed. The first essays of the opticians to produce "high powers" were, as might be expected, feeble. These powers amplified, but did not analyze; hence it began to be questioned whether "one could see more really with a high power than with a moderate one." And this was true at the time. But it is not so now. The optician has risen to the emergency, and provided us with powers of great magnifying capacity which carry an equivalent capacity for analysis. They open up structure in a wonderful way when rightly used. The lecturer began by projecting upon the screen the magnified image of a wasp's sting--an object about the 1-20th of an inch in natural size--and beside it was placed a piece of the point of a cambric sewing needle of the same length, magnified to the same extent. The details of the sting were very delicate and refined, but the minute needle point became riven and torn and blunt under the powerful analysis of the lens, showing what the lecturer meant by "magnifying power;" not mere enlargement, but the bringing out of details infinitely beyond us save through the well made lens. This was further illustrated by means of the delicate structure of the _Radiolaria_, and still further by means of a rarely delicate valve of the diatom known as _N. rhomboides_. With a magnification of 600 diameters no structure of any kind was visible; but by gradually using 1,200, 1,800, and 2,400 diameters, it was made manifest how the ultimate structure of this organic atom displayed itself. But this power of analysis was carried still further by means of the minutest known organic form, _Bacterium termo_. The lecturer had, in connection with Dr. Drysdale, discovered that the movements of this marvelously minute living thing were effected by means of a pair of fine fibers or "flagella." These were so delicate as to be invisible to everything but the most powerful and specially constructed lenses and the most delicate retinas. But since this discovery, Dr. Koch, of Germany, had actually photographed the flagella of much larger bacteria, such as _Bacillus subtilis_, and expressed his conviction that the whole group was flagellate. Mr. Dallinger determined then to try to measure the diameter of this minute _flagellum_ of _B. termo_ that the real power of magnification in our present lenses might be tested. This was a most difficult task, but 200 measurements were made with four different lenses, and the results were for the mean of the first 50 measurements 0·00000489208; for the second, 0·00000488673; for the third, 0·00000488024; for the fourth, 0·00000488200, giving a mean value for the whole, expressed in vulgar fractions, of the 1/204700 of an inch as the diameter of the flagellum of _B. termo_.