(_a_) _Compound microscope_.—Equipped with apochromatic objectives and
compensating oculars, giving magnifications of approximately 90, 180,
and 500 diameters. These magnifications can be obtained by the use
of 16 and 8 mm. Zeiss apochromatic objectives with X6 and X18 Zeiss
compensating oculars, or their equivalents, such as the Spencer 16 and
8 mm. apochromatic objectives[5] with Spencer X10 and X20 compensating
oculars, the draw-tube of the microscope being adjusted as directed
below.
(_b_) Thoma-Zeiss blood counting cell.[2]
(_c_) Howard mold counting cell.—Constructed like a blood-counting
cell but with the inner disk (which need not be ruled) about 19 mm. in
diameter.[6]
[5] We are informed that the Bausch and Lomb Optical Co. also furnishes
suitable apochromatic objectives and compensating oculars for use in
counting molds, yeast and bacteria by the Howard method.
[6] Comment by authors: In using these cells the plane parallel cover
glasses furnished with them by maker should be used instead of the
ordinary microscope cover-glasses, since the latter are subject to
curvatures that introduce errors in the thickness of the mounts.