| Molds | 66 per cent of fields. |
| Bacteria | 100 million per cubic centimeter. |
| Yeasts and spores | 125 per 1/60 cubic millimeter. |
These figures, of course, apply to the Howard method as employed by the Bureau of Chemistry. The method is entirely arbitrary and results agreeing with those obtained by the Bureau of Chemistry can be obtained only by using this method substantially as it is used by the bureau. An examination of the pulp, therefore, by an analyst who is not thoroughly conversant with this method as it is employed by the Bureau of Chemistry not only is useless but may actually afford a manufacturer a false sense of security which will be greatly to his disadvantage.
Microscopic Equipment Required
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.
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.
Optical Apparatus for the Howard Method
| Quantity desired | Item | Bausch & Lomb | Spencer |
| 1 | Microscope without oculars, objectives or other accessories | FF | 44 |
| 1 | Abbe condenser with two iris diaphragms (lower and upper) | 1740 | 300 |
| 1 | Double nosepiece | 1844 | 450 |
| 1 | 16 mm. achromatic objective | 1021 | 108 |
| 1 | 4 mm. achromatic objective with working distance of 0.6 mm. | 1029 | 116 |
| 1 | 8 mm. achromatic objective with working distance of 1.6 mm. | 1027 | 112 |
| 1 | 10X Huyghenian ocular | 1104 | 142 |
| 1 | Mechanical stage | 2116 | 485 |
| 1 | Substage lamp with Daylite glass | 1774 | 385-B |
| 1 | Blood counting chamber (Haemacytometer with ruling of Thoma, Neubauer, Jappert, Brewer or Turk) | 3550 | 1472 |
| 6 | Cover glasses for same, 20×21 mm., 0.4 thick | 3595 | 1460 |
| 1 | Howard’s mold counting chamber (with ¾ inch inner disk) for same | 3566 | Special |
| 6 | Cover glasses for same 33 mm. square, 0.6 mm. thick | 3598 | Special |
| 2 | Cases for counting chambers | 3580 | 1505 |
All analysts undertaking the Howard method should secure copies of the two bulletins of the United States Department of Agriculture written by Mr. B. J. Howard—Bulletin 569 on Sanitary Control of Tomato Canning Factories and Bulletin 581, Microscopic Studies on Tomato Products. These bulletins may be obtained from the Superintendent of Documents, Government Printing Office, Washington, D. C., on payment of five cents each in coin.