lenses is that adopted by Steinheil long before, in the construction of his well-known hand-magnifier (see page 77, Fig. 51). Zeiss’s 3 mm. objective has a triple front, balanced by two triple backs--in all nine lenses--a somewhat amplified diagram of which is represented in Fig. 118. The formula for this combination was furnished by Tolles, of Boston, America, and it at once secured increase of aperture (the value of this optician’s many contributions to microscopy has since his death been generally acknowledged). The metrical equivalent focus assigned by Zeiss to his series of dry achromatic objectives is given in somewhat ambiguous terms, which tend to confuse rather than classify them; for instance, two lenses of the same aperture--24 mm. and 16 mm.--corresponding to the English 1-inch and 2/3-inch, each have assigned to them an aperture of 0·30; a 12 mm. and 8 mm., corresponding to the English 1/2-inch and 1/3-inch, have an aperture of 0·65; while a 6 mm. = 1/4-inch, and a 4 mm. = 1/4-inch and 1/6-inch, have each an aperture of 0·95. Nachet exhibited at the Antwerp Exhibition a fine 1/10-inch oil-immersion, which was highly praised by the jurors. It is necessary, to make the fact perfectly clear, that dry and immersion lenses having the same angular aperture have also a similar defining power. The pencil of rays, however, differs in intensity and density as the rays emerging from the cover-glass of the object into air are very considerably deflected, and the cone suffers a corresponding loss of brightness. On this important point, then, I believe it will prove of value to interpolate a clear and full exposition of the change brought about by the cover-glass.