The apparatus employed by the Bureau of Chemistry includes apochromatic
objectives and compensating oculars. In 1914 it became impossible to
obtain these accessories[4] because of the European war and equivalent
apparatus of American manufacture was found to give the same results.
Both of these forms of apparatus are recognized in the official Howard
method which is given below.
[4] Apochromatic objectives may now be obtained but are more expensive
than the achromatic.
This laboratory made a careful study of the accessories available in
order to determine what could best be used. It was found that very
satisfactory results could be obtained by employing a 10X Huyghenian
ocular and a 4 mm. achromatic objective (working distance 0.6 mm.) and
a 16 mm. achromatic objective. These accessories require a careful
adjustment of light, but with proper use enable an analyst to secure
satisfactory results. It is found that the best results are obtained
with a rather dark field.
The apparatus necessary for the Howard method, including the
accessories mentioned above, may be obtained of two American
manufacturers, the Bausch & Lomb Optical Company, of Rochester, N. Y.,
and the Spencer Lens Company, of Buffalo, N. Y.
There is given below a full list of the optical apparatus required,
including catalog numbers of the two manufacturers, as far as numbers
have been assigned by them to the various items. In addition to the
apparatus given in this list, the analyst should have a 50 c. c.
graduated cylinder for measuring and diluting samples. This may be
obtained of any dealer in chemical apparatus and at many drug stores.
When ordering the optical apparatus the full description as given below
should be included.